Geez, newly matched Diamond here. Tried logging into hyatt.com and it either doesn't let me...or after several tries, lets me log in and then the moment I try to look up anything, it logs me out again and/or indicates "Session Timeout" for no good reason.

Unbelievable. I can't make any new reservation or reivew any of my existing reservations.

Not exactly a ringing endorsement. Reading the comments here, I can see this isn't a new phenomenon. The fact that Hyatt.com was down for 3 full days in Dec 2015 isn't reassuring, either.