SUMMARY
If there is no sound device, plasma_session crashes on login. It has more chance of happening if you have set a login sound in Plasma.

BACKTRACE
Application: plasma_session (plasma_session), signal: Segmentation fault

[New LWP 245567]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
0x00007f6be1bc6db0 in __lll_lock_wait () from /usr/lib/libpthread.so.0
[Current thread is 1 (Thread 0x7f6bde88f840 (LWP 245398))]

Thread 2 (Thread 0x7f6bddb2e640 (LWP 245567)):
[KCrash Handler]
#4  0x00007f6be340b090 in QObject::thread() const () at /usr/lib/libQt5Core.so.5
#5  0x00007f6bdd3171d1 in  () at /usr/lib/qt/plugins/phonon4qt5_backend/phonon_gstreamer.so
#6  0x00007f6be33e42ad in qAddPreRoutine(void (*)()) () at /usr/lib/libQt5Core.so.5
#7  0x00007f6be3b172de in call_init.part () at /lib64/ld-linux-x86-64.so.2
#8  0x00007f6be3b173c8 in _dl_init () at /lib64/ld-linux-x86-64.so.2
#9  0x00007f6be2eb60e5 in _dl_catch_exception () at /usr/lib/libc.so.6
#10 0x00007f6be3b1b705 in dl_open_worker () at /lib64/ld-linux-x86-64.so.2
#11 0x00007f6be2eb6088 in _dl_catch_exception () at /usr/lib/libc.so.6
#12 0x00007f6be3b1af3e in _dl_open () at /lib64/ld-linux-x86-64.so.2
#13 0x00007f6be13d034c in  () at /usr/lib/libdl.so.2
#14 0x00007f6be2eb6088 in _dl_catch_exception () at /usr/lib/libc.so.6
#15 0x00007f6be2eb6153 in _dl_catch_error () at /usr/lib/libc.so.6
#16 0x00007f6be13d0b89 in  () at /usr/lib/libdl.so.2
#17 0x00007f6be13d03d8 in dlopen () at /usr/lib/libdl.so.2
#18 0x00007f6be33dc9a9 in  () at /usr/lib/libQt5Core.so.5
#19 0x00007f6be33d6326 in  () at /usr/lib/libQt5Core.so.5
#20 0x00007f6be33d6913 in  () at /usr/lib/libQt5Core.so.5
#21 0x00007f6be33c7878 in QPluginLoader::load() () at /usr/lib/libQt5Core.so.5
#22 0x00007f6be36ac446 in  () at /usr/lib/libphonon4qt5.so.4
#23 0x00007f6be36add08 in  () at /usr/lib/libphonon4qt5.so.4
#24 0x00007f6be36ae1ae in Phonon::Factory::backend(bool) () at /usr/lib/libphonon4qt5.so.4
#25 0x00007f6be36ae7f7 in  () at /usr/lib/libphonon4qt5.so.4
#26 0x00007f6be36a3a5d in  () at /usr/lib/libphonon4qt5.so.4
#27 0x00007f6be36a1b48 in  () at /usr/lib/libphonon4qt5.so.4
#28 0x0000560dc4118ac5 in  ()
#29 0x00007f6be31fae8f in  () at /usr/lib/libQt5Core.so.5
#30 0x00007f6be1bbd3e9 in start_thread () at /usr/lib/libpthread.so.0
#31 0x00007f6be2e7b293 in clone () at /usr/lib/libc.so.6

Thread 1 (Thread 0x7f6bde88f840 (LWP 245398)):
#0  0x00007f6be1bc6db0 in __lll_lock_wait () at /usr/lib/libpthread.so.0
#1  0x00007f6be1bbf7d1 in pthread_mutex_lock () at /usr/lib/libpthread.so.0
#2  0x00007f6be3b17537 in _dl_fini () at /lib64/ld-linux-x86-64.so.2
#3  0x00007f6be2dbadb7 in __run_exit_handlers () at /usr/lib/libc.so.6
#4  0x00007f6be2dbaf5e in  () at /usr/lib/libc.so.6
#5  0x00007f6be2da3159 in __libc_start_main () at /usr/lib/libc.so.6
#6  0x0000560dc41110ce in  ()
[Inferior 1 (process 245398) detached]

Ugh, how I wish Arch Linux had a debug symbols option....

STEPS TO REPRODUCE
1. Log in. It's random, and happens 1/1000 times. Hard to reproduce.

OBSERVED RESULT
Crash handler as soon as the login logo goes away.

EXPECTED RESULT
Nothing.

SOFTWARE/OS VERSIONS
Windows: None
macOS: None
Linux/KDE Plasma: 5.4.61-rt37-1-rt-lts #1 SMP PREEMPT_RT Sat, 12 Sep 2020 21:01:23 +0000 x86_64 GNU/Linux
(available in About System)
KDE Plasma Version: 5.20.1
KDE Frameworks Version: 5.75.0
Qt Version: 5.15.1

ADDITIONAL INFORMATION
You need to disable PulseAudio autospawn and use JACK configured to auto-start on login using Cadence.
Trying to reproduce on 5.20.2 would take years, honestly. It's a very very rare bug.
I totally forgot how did I set JACK up to run on startup...