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}.
To resolve: