When users are trying to create an account, if their username has an uppercase letter it wont be able to create the account.
I only realized this was the reason I couldn't create an account after inspecting the webpage an seeing that the POST request to
https://kbase.us/services/auth/login/create/
was returning:
error | { httpcode: 400, httpstatus: "Bad Request", appcode: 30010, … }
httpcode | 400
httpstatus | "Bad Request"
appcode | 30010
apperror | "Illegal user name"
message | "30010 Illegal user name: Illegal character in user name DrJzzz2: D"
...