I too am having an issue with certain files uploading. I tried the web interface and the next cloud sync client with similar results. I’m actually running 21.0.3 now (upgraded to try to fix this) - and I disabled all extra apps, even went so far as to do a app:remove and app:install for every app that was enabled (most errored that they were shipped apps) - I even disabled all addons and still have the issue. I was running wonderfall docker images, but the repo apparently stopped being updated and I got behind in version. I switched to nextcloud official docker to upgrade from 19 to 20 to 21, still had the error, so I moved back to the new wonderfall image, still with the same result.
Attached are my pruned logs (lots of repeats, this just grabbed a few failures in this file)
(I guess I can’t attach files?) I didn’t even find the filename I was trying to upload in the logs, so I’m not sure what to copy/paste - but some files work, others do not, even within the same filetype.