Welcome to the forum @guaycuru
More information would help. Unfortunately, Duplicati doesn’t give all the details in default messages.
Does the backup get far enough to make a regular job log that names the extra files in the warnings?
Viewing the Duplicati Server Logs is another way to get info at About → Show log → Live → Warning
You can probably either start a backup (which will stop fast due to those) or try Verify files button.
When you find out the names of the extra files, see if they have the usual namings, and find the dates.
If it looks like Duplicati made them, look at the backup done at that time. Anything unusual about that?
If need be, use log-file=<path> and log-file-log-level=Information (or higher) to watch uploads and fails.