What plan do you have? If it's prepaid, which I have, the T-Mobile is no longer available. If it's a post-paid account I don't know what to advise you. I would suggest 611 (Customer Service) or T-Force. I have read that some have uninstalled the app and did a reinstall. About 3 weeks ago I had the app, and it worked perfectly. Someone must have made a decision to remove it from prepaid. The my T-Mobile site works for me, so far.
This is a new one for me. I remember seeing an issue posted internally that discussed this but that was a few months ago and it has since been resolved. Have you talked to your TEX (Team of Experts) yet? If not, we need to get this put on our Engineer's radar as this may or may not be a one-off situation. You can get in touch with them by dialing 611 or you can work with T-Force via Facebook / Twitter - whichever is most convenient.
You're not alone. I have a post-paid account, and cannot access any account details or billing, I can only log in. It's been like this for months. Today it actually has a new twist, instead of telling my it's having troubles loading the requested info, it just redirects me right back to the log-in screen with no error reports.