Upgrade to 8.03 Pro - Upload errors

Dear all,

I recently upgraded from 7.1.5 to 8.03 and I am now facing an issue where not all files get uploaded.
Trying to upload the same amount of files, again and again, different files fail from time to time.
The logs are showing nothing as well as my nginx logs.
the only feedback I get is:

I already tried a fresh install on another system (Ubuntu 20.04) and have the same issue.
Installing 7.1.5 again everything works. Upgrading or fresh installing 8.02 or 8.03 breaks seom of the uploads.
Also tried MySQL and sqlite backends, no change.

Any hint would be great.