From what I can understand about phpMyAdmin, a user is required to first connect to the database to get some information before logging in with the other user. In the code there are the following comments:
// Try to connect MySQL with the control user profile (will be used to
// get the privileges list for the current user but the true user link
// must be open after this one so it would be default one for all the
The error message you get looks like it is related to the control user profile.