Nerd hat on. After updating my Opera browser yesterday I had major problems logging into Gmail. After 12 hrs I've discovered the 413 error I kept getting was a browser fault. The fix is to copy & paste opera:config#Network|UseSpdy2 into your address bar & press enter. This should get you the relevant section of Opera Preferences Editor. Uncheck "Use Spdy2" & "Use Spdy3" & save. Then reboot the browser & everything's back to normal. Nerd hat off.