Hi Albert,
The bug I recalled was from 4.5, located here: http://support.dotnetnuke.com/issue/ViewIssue.aspx?id=5011&PROJID=2
Read the comments, especially those by Charles. When I do any QA testing, I use a freshly downloaded package. However, if you've been running DNN for awhile, it's very possible that your e-mail validation regex was not properly updated, and is still in a "pre-4.5" mode. This would explain why you experience the error and I don't.
Bottom line: check your Admin/Users/User Settings/EMail Validation regex. I bet it's out of date.
If this is so, you might consider opening up a new work item. This expression really should have been updated automatically for you during upgrade (if it's still set to an "old" default). The fact that we had to go back and forth on this issue is evidence enough that it creates confusion.
Brandon