When users are entering an invalid user name you must specify what the error is.

This message "Please, enter a valid username" is equal to the famous "Invalid" error message
the history tells us from 1970 :D
This is 40 years later and the "Invalid" message is still being coded

I can understand that you have limitations in order to use user names in links
but hide that programming trick from the user interface
let the users register with their real name and strip any illegal characters off the real name
and save it without display to the user or better use the userÍd for your links