There are many reasons when a username may be incorrect. We catch those, but the error message we provide is misleading. For example, when a dot is used, we still say 'Username containing an unescaped at-sign" (see screenshot). 

Rephrase accordingly and make it clearer to the user. Perhaps, something along the lines of:

"The username  is invalid. Usernames can only contain ASCII letters, numbers, hyphens, and underscores, and should begin with a letter or number"