This is my first post and I though I'd offer a solution to a problem I encountered.
I was totally unable to register via client v4. After I'd enter all the required information and hit OK button, there would be a long pause - around a minute I guess. Then I'd get the error message popup telling me my username had already been taken. No matter how creative I was, it'd give me the same message.
I solved it with registering via Curseforge.com board, only then the client allowed me to sign in.
I don't know if anyone had that same problem, but well, here's the solution.