On a multisite network I manage, I was just notified by a blog owner that users cannot register to their blog. I checked and it seems valid email addresses are being rejected as a username saying it needs to have only lowercase letters and numbers. WordPress itself does not enforce that policy as it makes no sense whatsoever anyway (email addresses are ideal usernames) and it worked before. I don't have a plugin enforcing such a policy either.
So I tested and reverted back version 1.4 of the User Registration addon and things work normally again.
Is there a specific reason version 1.5 now rejects email addresses as usernames? To me that is a bug as there's no reason I can think of that would justify this and this major change is also not mentioned in the 1.5 changelog that I can see.