I just had this happen to me a couple of hours ago. Here is a brief timeline...

2/5/11 - Upgraded perfectly working ZCM 10.3.1 to ZCM 11. For the rest of the day run tests to make sure all is well, tests all check out fine.

2/6/11 - Remote in and test again, making sure all is fine. Tests all check out fine.

2/7/11 AM - First thing in the morning, re-run tests of ZCM10 and 11 clients against upgraded ZCM11 environment. All tests check out fine again.

2/7/11 noonish - Getting errors from ALL users, 10's and the few 11's I've upgraded, that they can no longer logon to Zen. I verify this on my battery of test machines with the blank Realm fields in the ZAA logon prompt.

I then checked ZCM11 AD config and confirmed the "username/password" authentication mechanisms was blank. Reconfirming this setting and manually refreshing the ZAAs/rebooting the clients all resolved it.

OK, with that all now being the case, I'm a bit freaked out by it. Is this a known bug? Is this going to happen again randomly in the future? If so is there any way I can avoid it in the future? Is it a one-time thing, where once you re-set the setting it really will stay set for good?