The only thing I can think of, and given how thorough you've been, I'm probably grasping at a straw here, is that the passwords are case sensitive. If you're using the password you got from the reset, I know it's the proper length and doesn't contain invalid characters, so that's not it.
I'm going to ask our tech admin to step in, and hopefully she can assist where I cannot. Thanks for your patience - we will get this fixed for you!