Welcome to the forum @Stefan
Viewing the Duplicati Server Logs in About → Show log → Live → Retry would be an easy start, however setting up a log-file is possible if the problem is rare enough that you don’t want to watch.
–log-file-log-level can set its level. Retry might be fine here too. Highest level (BIG log) is Profiling.
If you’re very technically inclined on networking, Wireshark does well because FTP is unencrypted.
I have not, but some previously contributed examples of using net use for this were in posts below.
Cannot access network folder
UNC source with credentials?
Yes, although parallel has been talked about. Actually having it happen doesn’t seem likely real soon. Priority should be given to getting what’s current there working well enough that it can get out of beta.