So there is something very strange happening. I don't have two users, I'm pretty sure I rename it. I can log in via caipirinhapilot after resetting the password and the account is properly linked to steam, but when I log into the server via caipirinha_pilot, it causes an internal server error.
The question is: What happens when you rename an user? What happens to the old name?
The error message says where the problem is, I think the requirement to link the account (and having to relink it and confirm that it is linked) is the problem for most users. Having to mess around with steam and having to set everything to public, there are many steps that are probably too much for some users.
I notice that in here I'm called Caipirinha_Pilot, even though I'm logged in with the Capirinhapilot name.