How to fix "MySQL fails (username not known)"?

MySQL has limited usernames to 16 characters for many years. 

In versions of MySQL prior to 5.1, when a login username longer than 16 characters was sent to the server it was silently truncated to the 16 characters limit. 

In MySQL 5.1, all characters submitted for the username are checked during authentication. This change means that any user submitting more than 16 characters as their login username will fail to authenticate once the server is upgraded from MySQL 5.0.x To correct this problem, only send the first 16 characters of the login username.

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