No Data Received – Error on log in

The last few days I’ve had an issue trying to log into the dashboard of my site. The front end of the site works fine, looks like it should. However when I try to log in with my established username and password or try to register as a new user, I am brought to the expected page but there is no information only an error saying that no data was received.

None of my other wordpress sites are having this issue. I’ve updated wordpress to vs. 4.0 and downloaded and overwritten (via ftp) the theme files. I’ve changed the name of the plug in folder via ftp as well. Neither of these has made a difference.

Suggestions?