unable to log into iTunes Store no matter what iTunes account I use

Using iTunes 12.1 on OS 10.9.5, I cannot log into any iTunes account. I will enter a correct username and password and it will seemingly accept it, but a moment later it will ask for my username and password again, and no matter how many times I enter it, it will ask for login information again. This only happens on my primary (administrator) account on my Mac. If I log into a different user account on my Mac or log into using a different device (e.g. iPhone), I can log into the iTunes account just fine. This symptom happens no matter what iTunes account I try to log into, so there is an issue with the Mac user account.

So, I need to know what to do so I can log into the Store using my main account on my Mac.

I have already tried the following:

logging out of iTunes store and logging back in

Restarting the computer

Clearing the iTunes store cache from iTunes Preferences

Please help!