Sorry for the delay in responding to this, somehow we missed this post.
I'm not sure why you're having a continuous redirect to the login page. To get an idea of the flow causing issues:
* Create account
* Login
* ForcePassword redirects you to your details page
* On hitting save, it seems like ForcePassword redirects you again to your details page
If you disable the plugin, does logging in work as expected? The way FPC works is by looking at the users' last visit date to see if it's set to "0000-00-00 00:00:00", meaning they've never visited. Can you test a few different cases with and without the plugin enabled to see if the last visit date is changing after the user first logs in, and when it's actually changing? If it stays at 0000, then FPC is acting normally, but something's obviously up as that date should change.
Thanks!