Not sure if this is the right place to report this. Installed clean Ubuntu 20.04, converted to KDE via kubuntu-desktop. Installed Kdenlive 20.04 from the Snap Store. Put together a short test video of a few seconds of mp4 and a couple of pngs. Going to Render just gives the message "Waiting". It never renders. Uninstalled the snap and installed 19.12.3 (the deb) from the Snap Store. Put together the same test video, and this renders fine. So avoid the snap! (Though the interface is nicer in 20.04.) Also worth noting that the Snap Store tags the deb as having a proprietary license, which it doesn't (and this error seems to apply to a several items, eg Zim desktop wiki has the same mistagging.
Please try with the current Kdenlive AppImage version 20.08.1 to see if there are any packaging issues https://files.kde.org/kdenlive/release/ If the problem/issue doesn't occur when using the AppImage, then it's your configuration or packaging.
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This bug has been in NEEDSINFO status with no change for at least 30 days. The bug is now closed as RESOLVED > WORKSFORME due to lack of needed information. For more information about our bug triaging procedures please read the wiki located here: https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging Thank you for helping us make KDE software even better for everyone!