I don't think it has anything to do with being free. I made my account a few days ago and paid for 3 months. And I had the same problem. I logged in twice with my password with no troubles. Then it kept telling me my password was wrong and after 6 tries locked me out of the site.
I had to contact support and wait 2 days for them to send me a new password. And so far that has worked fine. They told me they are working on the site and some things are being effected. So might have something to do with that. Sorry for not being much help, I'm still learning it all.