Blank page upon front end login

Hi,
When we try to log in via the Classifieds login page on the front end, you get a plain white page (after entering user credentials). The address in the address bar is <site url>/wp-login.php. If you manually enter the site URL and go back to the home page, you are logged in. I tried enabling full error reporting in .htaccess, to no avail. Logging in via the standard back end /wp-login.php works fine.

We recently changed our permalink structure (not sure if that has anything to do with the error or not, just thought it was a possibility).

Thoughts? Thanks