Unable to Log into my prepaid account -- loops asking for UserID & PW, over and over...

Same here and going on six months or more. Only difference is I am the primary on a post paid account. It makes zero sense other than to confirm there's some super sketchy stuff going on. I wish that was the extent of my concerns but alas, it's not. It's beyond asinine that no one is willing to accept responsibility and at least help us access our own accounts. 

Getting stuck in loops like that make me physically dizzy. You have probably already done this, but I always start with clearing browser history. I do the first clear with just the default boxes checked. If the issue persists, I will clear the history again, but check more of the options so that I know for sure that nothing is getting stuck and preventing access. You can also try to load the page in a different web browser. Edge and Chrome are usually the most reliable. 

My next thought is maybe there is an error with the login that is not popping up when it directs you back to the beginning. Have you tried going through the forgot password options? Even if you haven't forgotten it, I find that resetting it can help if something on the website side is not working. It just irons out the kinks and usually breaks the loop. 

If neither of those work, you should reach out to our customer support team because it could be that you are locked out of the account for too many attempts. Plus they can review more details or file a ticket if the root cause is something even deeper. Most people on Community like to send a direct message on Facebook or X by selecting the social media icon at the bottom of the screen. That gets you to the T-Force team and they are fabulous!

Hope this helps.