Cannot login to iTunes Store from iTunes 12.3

I updated to iTunes 12.3 earlier today. When I attempted to sync my phone, I was told that all apps would be removed, as I was no longer authorized. Clicking the Authorize button it presented as an option did nothing. I then attempted to use the Authorize This Computer menu option in iTunes - it also failed to register the selection. I signed out of iTunes from the dropdown at top right, but when I tried to sign back in, once again the click failed to produce any response. An app re-start and a computer re-start failed to do anything.

I deleted the iTunes app (by using the Get Info window to set permissions) and then re-installed with a fresh copy of the iTunes installer. Same behavior after installation and re-start.

My account is still good - I can see it and manage it on my iPhone and iPad.

Suggestions appreciated!

MacBook Pro (Retina 13-inch); 3 GHz i7 w/ 16 GB ram