I have this same problem. Here’s what I’ve done to work around the issue.
If you have renamed your login page to https://yoursite.com/login there will be a “login” dir in your cache folder created by WP Fastest Cache located at wp-content/cache/all/login. If you FTP into your site and delete that folder then you will be able to get a fresh copy of your login page and actually log in. Of course, this keeps being a problem every time you need to log in.
A more permanent (but more risky) option…
If you have the ability to change the perms on your dir then, rather than deleting the entire “login” cache dir you can just delete the index.html file in the login dir. Then you can set the permissions on the login dir to 000 (or something that prevents WordPress from writing to that dir. So from the command line you could say:
chmod 000 wp-content/cache/all/login
Then WP Fastest Cache wont be able to write any cache files to that dir. The risky part is that this might cause some errors depending on your PHP configuration.
The moral of the story… delete the cache dir for your login page, refresh, and then you can log in again.