Unable to Sign In in iTunes 12.3.1.23 on El Capitan (MBP)
MBP, El Capitan version 10.11.1, iTunes version 12.3.1.23: Opened up iTunes, clicked on Sign In, nothing happened. Went to iTunes Store Menu, and clicked Authorize This Computer, nothing happened. Clicked Sign In, nothing happened. I am able to sign in to iTunes on my iPhone just fine, so I know my Apple ID is valid.
Any suggestions?