Help - Changed Iterations and can not log back in?

Sounds like a good plan. I would also suggest trying to log in using different browsers, in case there is some kind of browser limitation that is causing the 600k PBKDF2 iterations to fail.

In addition, keep an eye on this other recent thread, which could be related to your issue:

 
Finally, I wouldn’t totally discount the possibility that you may not be using the correct username/master password combo.

P.S. If you ever logged in using the Chrome browser extension, please see the advice I gave here. If you find your keyHash value and you know most of your master password, you may be able to get the rest using brute-force guessing.