Hello @alternatepersona and welcome to the forum!
A Google search for “SpillCollectorProcess” “FileNotFoundException” found this report, but also found
Backup error - sftp - put where the original poster thought their drive filled but Duplicati hadn’t noticed it. Whether or not that thinking will change is still TBD, however nothing in the past 3 days has changed it.
Is there free space, is this easily reproducible, and what version of Duplicati is it? How does the server compare with others that are not producing this error? As a side note, Server 2008 originally shipped a version of .NET Framework that’s too old for Duplicati, but I hope such an error gets a proper message. January 14 next year is when Windows Server 2008 itself reaches End of Support at Microsoft I believe.
If you really have to trace it down to see the activity for such dup-* Temp files, Process Monitor can do it. Getting this to error on a small backup will be easier than a long study in search of a particular file’s use.