My computer recently updated to the latest major upgrade of Windows 10, nicknamed by Microsoft the "Creators Update", version number 1703. Since then, I've had on and off problems with Retrospect not using Open File Backup. The last time I had the problem, I needed to reboot the computer to get it to work again. It worked for the next 3 backups, and then the error has just returned.
[*] MapError: unknown Windows error -1
[*] VssWEnumWriterMetadata: GatherWriterMetadata on tempBackupComp failed. Aborting writer exclude, osErr -1, error -1001
Or
[*] MapError: unknown Windows error -1
[*] VssWSnapVolume: PrepareForBackup failed, osErr -1, error -1001
Can't use Open File Backup option for Data on DATA (E:), error -1001 (unknown Windows OS error)
I'm using 12.1.0.174 Professional.