I had the same problem. None of the suggested solutions worked for me. However later I figured out something.
I had this plugin “WP Super Cache” installed and enabled for caching which is somehow not compatible with Premium content feature of Powerpress. When I turned off Caching by going to “WP Super Cache Settings” and setting “Caching Off” under the Easy tab, and the login started working. I checked in all the browsers and it is now working.
Hope this helps
Thanks
