iTunes won't let me log in or authorize computer

I seemed to have suddenly lost some functionality when I connect my iPhone to my desktop. Photos won't load the pictures on my iPhone, and on iTunes and I get odd errors when I try to sync my music (like that there's not enough space when there clearly is).

If I go to authorize my computer, here's the series of events:

1) Account --> Authorizations --> Authorize this computer

2) I enter my username and password and click "authorize"

3) My iPhone will alert me that my Apple ID is being used. I hit allow and receive the 6 digit verification code.

4) iTunes prompts me for my login and password again, with a new message saying the verification code screen will follow.

5) I enter my username and password and click "authorize"

6) My iPhone alerts me again and gives me another code

7) Meanwhile on iTunes, I get the message: "Did you forget your password?" followed by a note saying it was entered incorrectly more than once (it wasn't).

Most everything else I try doesn't get a response at all. For example if I "check for downloads" or try to update my iTunes, I'll get the same alert on my phone but nothing happens on my desktop.

Any ideas for fixes?

I'm running OS X Yosemite (version 10.10.5) on a Mid 2010 Mac Pro. iTunes is 12.7.2.58

My iPhone is an SE with up-to-date software.

I've tried restarting both the desktop and the iPhone. I even tried different wires and USB ports but no luck. I've never had this issue and I appreciate any suggestions.