Password won't work, even after resetting it.

I do not know if this applies to your case but there is a limit on how many different accounts you can log onto. After that point you need to reactivate those account, which requires you to have access to the mail accounts that they link to.

That might have been part of it, actually. The issue somehow managed to resolve itself, more or less, after a few days. My main account's ID no longer works, and I have to log in using the registered email, but the other accounts work again for some reason.

I'm not even going to try figuring out what was going on, it was really weird and Nexon didn't give me any answers in their reply to the ticket I sent. I can get on my account again at least, so that's good enough for now.