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.
fixed in  all login fields are now consistently restricted to maximum 100 characters string length + bundling DB updates
The change wasn't applied to core/Piwik.php, so fresh installs after 0.2.32 still create 20 character login fields.
(In ) fixes #522 - inconsistency in login string length
(update script to be addressed in #894)