Redmine

Hello,

I just build a new Redmine (Debian 10.10, Redmine 4.2.1).
Unfortunately, I am unable to upload any files in a ticket. I tried different Apps for Android (Redminer, RedminePM, Redmine Client) and for IOS (Kever), but any time I got the error "Error during upload".

If I try to upload a file with a web browser on the phone, everything is fine.
Does somebody have an idea, what the problem could be?

Thanks.