Some files of group folder do not get synchronized - Server reply "403 forbidden"

I moved an entire folder hierarchy from an standard (shared) folder to a new group folder. This was done via the Web Interface.

Most of the files then got synchronized correctly with the Windows Client to my local machine.

But for some of the files the client logged:
'“Server replied “403 forbidden” for GET [file_url]”

In fact there are no other permissions for these files than for the others.

Any ideas, what I can check?

After some more investigation on this, I found out, that password protected PDF files cause this behavior.

There is already another ticket on that issue (but without solution!):

Can you post the Nextcloud Server log entry associated with this download attempt?

Issue is solved.
When checking the server logs I found that the proxy might be the problem.
Ticket can be closed.
Thanks for help.