I have been upgrading this version on this windows 2012 r2 for quite some time and this is the first time I had the update actually fail.
I can repeat the failure after restoring the server subsonic dir back to the previous version.
One thing to note is the setup hangs for about 2 minutes at the start of the upgrade then continues and informs the "upgrade" was successful.
After starting the server and when I logon the setup did not upgrade but replaced the setup with a fresh 6.0 version.
I did notice the db backup dir is empty, the setup has none of the previous config and the licence is not carried forward.
If there is a debug flag for the setup I would be happy to run in debug mode to help troubleshoot.
Be forewarned the Windows setup might have issues so always backup before you attempt an upgrade!