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?
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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 
https://github.com/discourse/discourse/commit/e204144a582ec8c3a4424516b3b1b00d2b27655f
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