Here's what I found out about what happened to cause the "User Not Found" error this weekend.
The site was down due to an obscure bug, which has been fixed. The bug was there in the code for the last year but only surfaced when a specific data condition was met causing it to give a false error message (User not found). This has now been fixed in both code and data and will not occur again.
Sorry for any inconvenience this caused.