An unknown response null: :text/html; charset=utf-8: :Bad Request was received

[answer=42]

Check your logs folder for any additional errors generated and actually, the error was generated "by upgrading", but by not maintaining the involved software compatible with the current in use version of ZC. There are likely/possibly other actions necessary to further make the responsive template compatible, but that is an issue to address/discover within the thread for the particular software.

Thank you.
After checking logs there was an error; PHP Warning: constant(): Couldn't find constant includes/functions/admin_access.php on line 38
time for some research.