So, the issue is simple.
When I try to save user preferences with nickname vitalino4ka_ an error is thrown:
Username must end with a letter or number or an underscore
The username doesn’t look wrong, does it?
1 Mi Piace
The username validation changed a bit and unfortunately the error message didn’t follow up.
The rules are defined in the “UsernameValidator” class which states that the username must end with alphanumeric. (ie. no underscore)
I just pushed a fix 
2 Mi Piace