Thread:Project:Support desk/Upgrade from MW 1.18.1 to 1.23.5 broke sysop and Bureaucrat users functionality/reply (9)

I expect that the UserMerge extension works with the user ID, not with the user name. That means: It should work, but what you do is not what is supposed to be done. I would double check, if usernames are displaying correctly after that, e.g. in the page histories.

As for your usernames: MediaWiki username must start with a capital letter. That means that your user "username" is invalid. You should not have users with first letter in lowercase. As for the other variations, I think they might be OK. There is a maintenance script, which can check users for validity. You might want to run it to see...

Anyway, I think that the problem originates in the way you are doing the login/registration of user accounts for MediaWiki. MediaWiki should not change any user name. So "USername" and "Username" are two different accounts, but whether you supply the one or the other should not cause problems.