File server backup suggested another tuning (and more) for a 12 TB server. Your 5 TB might benefit.
The working backup seems like it must already have an oversize Remote volume size per job stats showing 898 GB on the remote. Because each dblock has a dindex, 2500 dblocks hold only 125 GB.
Viewing the log files of a backup job to find BackendStatistics KnownFileCount should either be under 5000 due to larger remote volume size, or somehow the Alle backup is working differently than others.
For the error ones, does it upload for a long time (depending on upload speed), then fail towards end? Uploading does not continuously read the directory listing, but file listing is checked after uploads end.
If you search for words onedrive list view 5000 you’ll see this is a notorious limit on file listings… There are some signs that the new Microsoft Graph API helps it, though I haven’t seen it documented.
Storage Providers on page right side Table of Contents shows which storage types use the Graph API. Generally I’d think Microsoft SharePoint v2 would be your best fit, but I can’t detail how to convert.
What storage type are you using now? OneDrive for Business uses SharePoint technologies, I believe.
Here is some talk about the 5000 list view limit. Results from these aren’t clear, but links from them are.
No backup without “–no-backend-verification=true” also gives some migration hints if you want to try…
Issue using Onedrive for Business
Various positive + negative feedback also talked about German servers being different. I know no more.