Table "piwik_user_dashboard" has got a login-attribute, which is limited to 20 chars.
I’ve set up an username with more than 20 chars and therefore the login-value was cut to 20 chars and I couldn’t change any settings in the dashboard. I went to phpMyAdmin and changed the login-attribute to 60 chars.

Piwik should throw an error if someone tries to setup an user with more than 20 chars or you should raise the char-length in the database.

Solution: we should make sure that login is restricted to the same length in the mysql tables (60 chars), and that it’s properly checked against when creating a new user.