I booked a multi-city MFU a few hours ago (over the phone) - I'm yet to receive an e-mail, but I could check everything in MMB 5 min after the booking... until at around 9pm it suddenly kicked me out.

I wasn't worried as it WAS showing on checkmytrip.com at the time... and like magic I can now see it back in MMB.

I'm only sharing this to reassure the OP that they weren't the only one who had an issue tonight.

EDIT: It's 23:10 (UK) and MMB isn't letting me in again. I guess this will be resolved in time... not that it's urgent for me, I'm not making this trip until 2010 :-)