I had nearly the exact same experience. There must be some kind of bug. I created a pseudononymous account using a Reddit login. A couple day later I created this account from which I am posting now using a Facebook login. All worked fine for a couple days. And then suddenly it started telling me Incorrect Password on my first account. I KNOW I'm using the right password. Like you I saved in in a password keeper. Plus I memorized it.

There has to be some type of bug when people register multiple accounts from the same computer.

Anyway, if you figure it out, please let me know. I will do the same.