My wife and I each have an HTC HD7 on a family plan with T-Mobile. I can access marketplace and music + video apps with no problems. However, when my wife tries to do the same on her phone, the connection times out and she is returned to the applications menu.
Any ideas? Is the kind of parental lockout in place?
I have experienced this problem as well. The temp. fix is to warm boot the phone. To do this press and hold the power button until you see "goodbye" on your screen. Than simply power it back up. That should fix the problem.