Can you share the name of the third-party module?  Aside from restoring from backup (which I realize isn't always available), I've had some success in simply waiting.  I had a similar problem with a site on GoDaddy shared hosting.  About 24 hours later the login feature worked... Very odd, but a happy ending.  Perhaps something to do with restarting the application?

Thanks Guys. It was the cookies that seemed to be causing the problem.  waiting may have helped too. I simply logged in on another workstation and the website was then accessable.