I'm also seeing this. According to FFMPEG, I have the ability to encode via libx264.

Platform:

 Debian sid

Packages:

ffmpeg/unstable,now 7:4.3-2 amd64 [installed,automatic]
  Tools for transcoding, streaming and playing of multimedia files

libavcodec58/unstable,now 7:4.3-2 amd64 [installed]
  FFmpeg library with de/encoders for audio/video codecs - runtime files

libx264-155/unstable,now 2:0.155.2917+git0a84d98-2 amd64 [installed,automatic]
  x264 video coding library


ffmpeg output:

$ ffmpeg -codecs
ffmpeg version 4.3-2 Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 9 (Debian 9.3.0-13)
  configuration: --prefix=/usr --extra-version=2 --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --arch=amd64 --enable-gpl --disable-stripping --enable-avresample --disable-filter=resample --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librabbitmq --enable-librsvg --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-opencl --enable-opengl --enable-sdl2 --enable-pocketsphinx --enable-libmfx --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared
  libavutil      56. 51.100 / 56. 51.100
  libavcodec     58. 91.100 / 58. 91.100
  libavformat    58. 45.100 / 58. 45.100
  libavdevice    58. 10.100 / 58. 10.100
  libavfilter     7. 85.100 /  7. 85.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  7.100 /  5.  7.100
  libswresample   3.  7.100 /  3.  7.100
  libpostproc    55.  7.100 / 55.  7.100
---
DEV.LS h264                 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 (decoders: h264 h264_v4l2m2m h264_qsv ) (encoders: libx264 libx264rgb h264_omx h264_qsv h264_v4l2m2m h264_vaapi )


I tried to set the "Bypass codec verification" option, and that seemed to have no effect. I also tried to re-run the config wizard a few times, and a green notification showed up saying the codecs had been updated, but also no joy.

Let me know what other information I can provide. This is seriously impacting my video production :-( -- the WebM output is terrible quality, even at the maximum.

I ran kdenlive from the terminal, and when I opened the Render dialog, this was the output in my terminal:

---
formats:
  - a64
  - ac3
  - adts
  - adx
  - aiff
  - amr
  - apng
  - aptx
  - aptx_hd
  - asf
  - ass
  - ast
  - asf_stream
  - au
  - avi
  - avm2
  - avs2
  - bit
  - caf
  - cavsvideo
  - codec2
  - codec2raw
  - crc
  - dash
  - data
  - daud
  - dirac
  - dnxhd
  - dts
  - dv
  - eac3
  - f4v
  - ffmetadata
  - fifo
  - fifo_test
  - filmstrip
  - fits
  - flac
  - flv
  - framecrc
  - framehash
  - framemd5
  - g722
  - g723_1
  - g726
  - g726le
  - gif
  - gsm
  - gxf
  - h261
  - h263
  - h264
  - hash
  - hds
  - hevc
  - hls
  - ico
  - ilbc
  - image2
  - image2pipe
  - ipod
  - ircam
  - ismv
  - ivf
  - jacosub
  - kvag
  - latm
  - lrc
  - m4v
  - md5
  - matroska
  - matroska
  - microdvd
  - mjpeg
  - mlp
  - mmf
  - mov
  - mp2
  - mp3
  - mp4
  - mpeg
  - vcd
  - mpeg1video
  - dvd
  - svcd
  - mpeg2video
  - vob
  - mpegts
  - mpjpeg
  - mxf
  - mxf_d10
  - mxf_opatom
  - null
  - nut
  - oga
  - ogg
  - ogv
  - oma
  - opus
  - alaw
  - mulaw
  - vidc
  - f64be
  - f64le
  - f32be
  - f32le
  - s32be
  - s32le
  - s24be
  - s24le
  - s16be
  - s16le
  - s8
  - u32be
  - u32le
  - u24be
  - u24le
  - u16be
  - u16le
  - u8
  - psp
  - rawvideo
  - rm
  - roq
  - rso
  - rtp
  - rtp_mpegts
  - rtsp
  - sap
  - sbc
  - scc
  - film_cpk
  - segment
  - stream_segment,ssegment
  - singlejpeg
  - smjpeg
  - smoothstreaming
  - sox
  - spx
  - spdif
  - srt
  - streamhash
  - sup
  - swf
  - tee
  - 3g2
  - 3gp
  - mkvtimestamp_v2
  - truehd
  - tta
  - uncodedframecrc
  - vc1
  - vc1test
  - voc
  - w64
  - wav
  - webm
  - webm_dash_manifest
  - webm_chunk
  - webp
  - webvtt
  - wtv
  - wv
  - yuv4mpegpipe
  - chromaprint
  - alsa
  - caca
  - fbdev
  - opengl
  - oss
  - pulse
  - sdl,sdl2
  - sndio
  - video4linux2,v4l2
  - xv
...
---
audio_codecs:
  - comfortnoise
  - dvaudio
  - 8svx_exp
  - 8svx_fib
  - s302m
  - aac
  - aac_fixed
  - aac_latm
  - ac3
  - ac3_fixed
  - acelp.kelvin
  - alac
  - als
  - amrnb
  - amrwb
  - ape
  - aptx
  - aptx_hd
  - atrac1
  - atrac3
  - atrac3al
  - atrac3plus
  - atrac3plusal
  - atrac9
  - bmv_audio
  - cook
  - dca
  - dolby_e
  - dsd_lsbf
  - dsd_msbf
  - dsd_lsbf_planar
  - dsd_msbf_planar
  - dsicinaudio
  - dss_sp
  - dst
  - eac3
  - evrc
  - wavesynth
  - flac
  - g723_1
  - g729
  - gsm
  - gsm_ms
  - hca
  - hcom
  - iac
  - ilbc
  - imc
  - interplayacm
  - mace3
  - mace6
  - metasound
  - mlp
  - mp1
  - mp1float
  - mp2
  - mp2float
  - mp3float
  - mp3
  - mp3adufloat
  - mp3adu
  - mp3on4float
  - mp3on4
  - mpc7
  - mpc8
  - nellymoser
  - on2avc
  - opus
  - paf_audio
  - qcelp
  - qdm2
  - qdmc
  - real_144
  - real_288
  - ralf
  - sbc
  - shorten
  - sipr
  - siren
  - smackaud
  - sonic
  - tak
  - truehd
  - truespeech
  - tta
  - twinvq
  - vmdaudio
  - vorbis
  - wavpack
  - wmalossless
  - wmapro
  - wmav1
  - wmav2
  - wmavoice
  - ws_snd1
  - xma1
  - xma2
  - pcm_alaw
  - pcm_bluray
  - pcm_dvd
  - pcm_f16le
  - pcm_f24le
  - pcm_f32be
  - pcm_f32le
  - pcm_f64be
  - pcm_f64le
  - pcm_lxf
  - pcm_mulaw
  - pcm_s8
  - pcm_s8_planar
  - pcm_s16be
  - pcm_s16be_planar
  - pcm_s16le
  - pcm_s16le_planar
  - pcm_s24be
  - pcm_s24daud
  - pcm_s24le
  - pcm_s24le_planar
  - pcm_s32be
  - pcm_s32le
  - pcm_s32le_planar
  - pcm_s64be
  - pcm_s64le
  - pcm_u8
  - pcm_u16be
  - pcm_u16le
  - pcm_u24be
  - pcm_u24le
  - pcm_u32be
  - pcm_u32le
  - pcm_vidc
  - derf_dpcm
  - gremlin_dpcm
  - interplay_dpcm
  - roq_dpcm
  - sdx2_dpcm
  - sol_dpcm
  - xan_dpcm
  - adpcm_4xm
  - adpcm_adx
  - adpcm_afc
  - adpcm_agm
  - adpcm_aica
  - adpcm_argo
  - adpcm_ct
  - adpcm_dtk
  - adpcm_ea
  - adpcm_ea_maxis_xa
  - adpcm_ea_r1
  - adpcm_ea_r2
  - adpcm_ea_r3
  - adpcm_ea_xas
  - g722
  - g726
  - g726le
  - adpcm_ima_amv
  - adpcm_ima_alp
  - adpcm_ima_apc
  - adpcm_ima_apm
  - adpcm_ima_cunning
  - adpcm_ima_dat4
  - adpcm_ima_dk3
  - adpcm_ima_dk4
  - adpcm_ima_ea_eacs
  - adpcm_ima_ea_sead
  - adpcm_ima_iss
  - adpcm_ima_mtf
  - adpcm_ima_oki
  - adpcm_ima_qt
  - adpcm_ima_rad
  - adpcm_ima_ssi
  - adpcm_ima_smjpeg
  - adpcm_ima_wav
  - adpcm_ima_ws
  - adpcm_ms
  - adpcm_mtaf
  - adpcm_psx
  - adpcm_sbpro_2
  - adpcm_sbpro_3
  - adpcm_sbpro_4
  - adpcm_swf
  - adpcm_thp
  - adpcm_thp_le
  - adpcm_vima
  - adpcm_xa
  - adpcm_yamaha
  - adpcm_zork
  - libcodec2
  - libgsm
  - libgsm_ms
  - libopus
  - libspeex
  - libvorbis
...
---
video_codecs:
  - h263_v4l2m2m
  - h264_v4l2m2m
  - h264_vaapi
  - hevc_v4l2m2m
  - hevc_vaapi
  - mjpeg_vaapi
  - mpeg2_vaapi
  - mpeg4_v4l2m2m
  - vp8_v4l2m2m
  - vp8_vaapi
  - vp9_vaapi
  - aasc
  - aic
  - alias_pix
  - agm
  - amv
  - anm
  - ansi
  - apng
  - arbc
  - asv1
  - asv2
  - aura
  - aura2
  - avrp
  - avrn
  - avs
  - avui
  - ayuv
  - bethsoftvid
  - bfi
  - binkvideo
  - bitpacked
  - bmp
  - bmv_video
  - brender_pix
  - c93
  - cavs
  - cdgraphics
  - cdtoons
  - cdxl
  - cfhd
  - cinepak
  - clearvideo
  - cljr
  - cllc
  - cpia
  - camstudio
  - cyuv
  - dds
  - dfa
  - dirac
  - dnxhd
  - dpx
  - dsicinvideo
  - dvvideo
  - dxa
  - dxtory
  - dxv
  - eacmv
  - eamad
  - eatgq
  - eatgv
  - eatqi
  - 8bps
  - escape124
  - escape130
  - exr
  - ffv1
  - ffvhuff
  - fic
  - fits
  - flashsv
  - flashsv2
  - flic
  - flv
  - fmvc
  - 4xm
  - fraps
  - frwu
  - g2m
  - gdv
  - gif
  - h261
  - h263
  - h263i
  - h263p
  - h264
  - h264_qsv
  - hap
  - hevc
  - hevc_qsv
  - hnm4video
  - hq_hqa
  - hqx
  - huffyuv
  - hymt
  - idcinvideo
  - iff
  - imm4
  - imm5
  - indeo2
  - indeo3
  - indeo4
  - indeo5
  - interplayvideo
  - jpeg2000
  - jpegls
  - jv
  - kgv1
  - kmvc
  - lagarith
  - loco
  - lscr
  - m101
  - magicyuv
  - mdec
  - mimic
  - mjpeg
  - mjpegb
  - mmvideo
  - motionpixels
  - mpeg1video
  - mpeg2video
  - mpeg4
  - mpegvideo
  - mpeg2_qsv
  - msa1
  - mscc
  - msmpeg4v1
  - msmpeg4v2
  - msmpeg4
  - msrle
  - mss1
  - mss2
  - msvideo1
  - mszh
  - mts2
  - mv30
  - mvc1
  - mvc2
  - mvdv
  - mvha
  - mwsc
  - mxpeg
  - notchlc
  - nuv
  - paf_video
  - pam
  - pbm
  - pcx
  - pfm
  - pgm
  - pgmyuv
  - pictor
  - pixlet
  - png
  - ppm
  - prores
  - prosumer
  - psd
  - ptx
  - qdraw
  - qpeg
  - qtrle
  - r10k
  - r210
  - rasc
  - rawvideo
  - rl2
  - roqvideo
  - rpza
  - rscc
  - rv10
  - rv20
  - rv30
  - rv40
  - sanm
  - scpr
  - screenpresso
  - sgi
  - sgirle
  - sheervideo
  - smackvid
  - smc
  - smvjpeg
  - snow
  - sp5x
  - speedhq
  - srgc
  - sunrast
  - svq1
  - svq3
  - targa
  - targa_y216
  - tdsc
  - theora
  - thp
  - tiertexseqvideo
  - tiff
  - tmv
  - truemotion1
  - truemotion2
  - truemotion2rt
  - camtasia
  - tscc2
  - txd
  - ultimotion
  - utvideo
  - v210
  - v210x
  - v308
  - v408
  - v410
  - vb
  - vble
  - vc1
  - vc1image
  - vc1_qsv
  - vcr1
  - vmdvideo
  - vmnc
  - vp3
  - vp4
  - vp5
  - vp6
  - vp6a
  - vp6f
  - vp7
  - vp8
  - vp9
  - vqavideo
  - webp
  - wcmv
  - wrapped_avframe
  - wmv1
  - wmv2
  - wmv3
  - wmv3image
  - wnv1
  - xan_wc3
  - xan_wc4
  - xbm
  - xface
  - xl
  - xpm
  - xwd
  - y41p
  - ylc
  - yop
  - yuv4
  - 012v
  - zerocodec
  - zlib
  - zmbv
  - libopenjpeg
  - librsvg
  - libvpx
  - libvpx-vp9
  - bintext
  - xbin
  - idf
  - libaom-av1
  - mjpeg_qsv
  - vp8_qsv
  - vp9_qsv
...

Also seeing this in Ubuntu 20.10 with ffmpeg 7:4.3-2ubuntu2 (4.3)

FYI, also: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=963706

Seems MLT just needed a no change rebuild against the new ffmpeg. I have done this for Ubuntu 20.10.

So this is not a bug in kdenlive.