We investigated this offline. The root cause was that the S3 storage service (strato hidrive) does not support multi-part copy. The workaround is to use the rsync format (Backups -> configure -> Format) when you have backups > 5GB.
thank you for your replies. I found the error in a complete different field. A custom emoji crashed the bulk export. After deleting this I could perform the export as planned and the import to Cloudron worked as described in the documentation. Still thank you for your efforts.