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BugZero found this defect 2930 days ago.
Windows File Level Restore hangs on "Starting restore session" for abnormally long period of time while free disk space on drive C:\ keeps decreasing dramatically on the machine where the Veeam Backup & Replication Console is launched. You may that find the following location of the cache file constantly growing: C:\Windows\system32\config\systemprofile\AppData\Local\mount_cache{GUID} or C:\Windows\SysWOW64\config\systemprofile\AppData\Local\mount_cache{GUID}.
The issue occurs when guest Windows has Transactional NTFS (TxF) turned on. When mounting to Veeam server it can cause NTFS driver to enter infinite loop trying to re-apply transaction log.
To resolve: Login to the guest OS of the machine being backed up as administrator. Open an Administrative Command Prompt Execute 2 following commands for each logical disk in VM.Example for disk C: fsutil resource setautoreset true C:\ fsutil resource setavailable C:\ Run the backup job to create a new restore point.Note: This will correct the issue for all future restore points, and is not retroactive. Test the Windows File Level Restore again
For more information about Transactional NTFS (TxF) https://docs.microsoft.com/en-us/windows/win32/fileio/transactional-ntfs-portal