Getting SyntaxError: JSON

Hi my backoffice has been working fine, however today I go to log into my admin account and I get this message: SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data (plus some other stuff)

Now every time I click to go anywhere it logs me out. I noticed that when I log back in I'm at the place I last clicked. Can anyone please help. It's odd because it just started doing this today.

Something must have changed for this to start happening. Have you installed any extensions recently?