MyBB Community Forums

Ok, my issue was that the usernames in the DB needed to be only UPPERCASE. 
The solution was to add a trigger to my mysql database.

Connect to the mysql database with correct access:
mysql -u root -p

Select the DB, in my case mybb
use mybb;

Then add the trigger for the username:
CREATE TRIGGER forceupper BEFORE INSERT ON mybb_users FOR EACH ROW SET NEW.username = UPPER(NEW.username);

In case of a mistake, remove the trigger:
DROP TRIGGER forceupper;

WHY must your usernames be uppercase in the database? Is it for integration with another system (eg. Mediawiki requires wiki-case)? The MyBB login code is case-insensitive.

Why not create a plugin which uses the datahandler_user_insert and datahandler_user_update hooks? You may want to see which other hooks are available in the UserDataHandler class ( inc/datahandlers/user.php).