Maybe I should mention that I have moved overseas in the meantime, so getting the phone to AT&T customer service is not an option, unfortunately. With the help of Google and also this forum I finally figured out how I can get to the boot menu to select Recovery mode. Unfortunately that doesn't help either. It takes a bit longer, I get a symbol on the screen that shows a phone and a triangle for some time and then I'm back in the old boot loop.
I'd be grateful if somebody could point me towards things I could try out next to recover at least the data I have not backed up (mostly pics and a few phone numbers). The hope for data recovery is also the reason I haven't tried the factory reset option yet.
Thanks and please ask if you need more information.
Here's the information from my boot menu:
*** LOCKED ***
M7_UL PVT SHIP S-ON RH
May 3 2013,19:16:59:-1