With 21.08.1 on ubuntu video proxys can't be generated. A message tells that after drag&drop a video to project. There is a button for "protocol", a click to that and e empty error message windows appears with OK button. Later, perhaps 10 minutes later, a error message appears, sometimes directly after including a wav file to project::

[h264 @ 0x5582e8a86d80] No support for codec h264 profile 100.
[h264 @ 0x5582e8a86d80] Failed setup for format vaapi_vld: hwaccel initialisation returned error.
[hwupload @ 0x5582e8c30380] A hardware device reference is required to upload frames to.
[Parsed_hwupload_1 @ 0x5582e8b1ad00] Query format failed for 'Parsed_hwupload_1': Invalid argument
Error reinitializing filters!
Failed to inject frame into filter network: Invalid argument
Error while processing the decoded data for stream #0:0

tia.

SOLVED: a new project has no problems with generating proxy videos: I opened a new project and all videos I drag&drop produce no error. My actual running project was created 2 months ago with version lower that 21.08.1, I don't know which version.
BTW: I tried to render a section or whole (old!) project: rendering hangs from beginning!!! PANIC!! 40 hrs work in this project!!!! I need to render!!!

How I "repaired" this project:
1. close kdenlive.
2. rename the folder with project files: myfolder -> myfolder-b
3. Start kdenlive, open project. kdenline claimed lots of missing clips because "myfolder" no longer exists
4. make recusive search in myfolder-b . Remove one mkv-clip from project that was no longer present, I deleted it 1 hr ago
5. kdenlive completed successfully.
Now proxy works again. Rendering works again

I hope this helps other kdenlive users!


But: proxy of videos are still NOT generated:
right mouse click on video in project -> proxy: Error message window:
[hwupload @ 0x5623f8b2e600] A hardware device reference is required to upload frames to.
[Parsed_hwupload_1 @ 0x5623f8a6b440] Query format failed for 'Parsed_hwupload_1': Invalid argument
Error reinitializing filters!
Failed to inject frame into filter network: Invalid argument
Error while processing the decoded data for stream #0:0