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_backups/IncrementalBackup_2013-08-09_1806/Databases/<folder name>/<database name>.fmp12" to "filewin:/D:/fmp_progressive_backups/InProgress_FMS/Databases/<folder name>/". (20402)
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.