I've tried via Web or SFTP oder FTP. Via SFTP or FTP the upload always stops at the same size (between 33 and 34 GB) and starts over again. It's not a Filezilla timeout, must have to do with the proxmox server.

Via web it gives me an error right at the beginning.

I cleaned up the old files and the disk is far big enough.

Has anyone an idea about that?

There is quite a lot of free space, more then 400 GB.
STFP, as I said, always stops at the same point / file size.
Unfortunately I'm not really familiar with rsync.

It was indeed a firewall issue. Disabling the firewall made the upload successful.

your own firewall or PVE firewall?

PVE firewall