Unable to backup iPhone to PC

Win8.1 PC & iPhone 5S.

iTunes & iPhone SW are up-to-date.

I've no interest in using the Cloud & want to backup my iPhone to my PC via iTunes.

I have the "This Computer" box checked under Backups in iTunes & iTunes recognizes my iPhone.

Music Playlists synch from iTunes-on-the-PC to the iPhone just fine when I synch (just purchased some songs, downloaded them to a playlist on the PC/iTunes then synched that list to the iPhone.

Every time I synch I get the "backup failed" msg from iTunes with no explanation.

When I try to manually backup the iPhone to the PC via iTunes, after a couple of minutes I get the error msg: "iTunes could not back up the iPhone [name of my phone] because the backup session failed." Duh - highly information, eh?

All iTunes Diagnostic test passed. The only odd thing I notice is the statement: "Current user is not an administrator" which makes no sense to me in that I'm the only user of this PC.

For years I've done this until the iCloud came to be last year Ever since, I've not been able to backup to my PC.