Progressive backup failure
Version: FileMaker Server 18.104.22.1685
Platform: Microsoft Windows Server 2008 R2
Error from logs (text between <> is redacted info): 2013-08-09 18:14:45.285 -0500 Error 791 <server name> Unable to proceed with progressive backup of "LacCore Core Database" until copy of database is manually moved from "file:/D:/fmp_progressive_
Recent changes: I recently moved the progressive backups to a different drive, a few days prior to the event.
I'm thinking Filemaker Server was having some issues copying files from one folder to the "InProgress_FMS" folder, I don't see this folder anywhere, so I'm assuming it's some sort of temp folder. In an attempt to reproduce the issue, I made a change to the database, but the progressive backup ran without problems, I verified by checking the modification dates of the folders where progressive backups are written to. Other databases had no issues with progressive backups during the time of the error.
What could the cause of this error be?