Installing and upgrading help

Can you be more specific... can you give us an actual example of what you are doing and when you get this message?

Usernames in Moodle are lowercase only and emails are case-insensitive anyway (not just in Moodle).

My recommendation is no optional plugins. That is until you ever identify a specific need. All optional plugins have the potential to give you pain at upgrading time.

I have seen this same error message when site has self enrolment enabled and admin tries to use bulk user actions to delete such users who have empty name fields (the whole name is empty). This kind of situations might occur when for some reason all data in user fields are not correct and moodle somehow still saves new user without a name (a person tries to create a new account with the same email that is already in use?). In moodle 2.5 the same bulk user actions worked ok.

So it is most likely a bug - and probably connected to https://tracker.moodle.org/browse/MDL-46232

I just checked that in my case those users causing failure in bulk user action deletion had empty email field. They had started to create a new account but had not finished all fields.

Do you have phpMyAdmin or some other tool to get direct access to database and table mdl_user ? It would be easy to check and change your admin email with it (can be done with sql commands as well)