504 Gateway Timeout When Trying To Access Backend

We can no longer log into the back end of the website - no idea why. Was all fine last night - today when trying to login I get a 504 Gateway Timeout.

This is where it gets weird. The website is still working, taking orders etc. And I can access the backend login screen - its only after my user and pass has been entered does it time out.


Started working again. No changes from me. Strange