Note that the same happens for .mov files as well. Also happens around 4%. Windows Firefox on Windows 11.

It shows that directory is missing, because it is actually a symlink.
lrwxrwxrwx 1 tools.video2commons tools.video2commons 36 Dec 26 2016 /data/project/video2commons/video2commons/frontend/static/uploads -> /data/scratch/video2commons/uploads/

In /data/scratch there is no video2commons directory. But there is a message:

This volume was recently migrated to a new, empty server.

If you had files here that you need, they can probably be recovered.  Open
a phabricator task and assign it to Andrew Bogott.

Also, in the future, don't put files that you need here! There is
no guarantee of persistence or reliability for the scratch volume.

It probably needs to be changed. I'm subscribing @Andrew as he can probably tell more.

Still broken btw.

I'm getting the same error message (Error: An exception occurred: FileNotFoundError: b"[Errno 2] No such file or directory: '/data/project/video2commons/video2commons/frontend/static/uploads/55f5972c-715e-11ed-a81e-66fb5a932327'") at the start of the upload. The progress bar keeps going to 100% but I'm not able to move on after that.