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

[answer=42]

Yes, that's your problem.
If you want to use that template on v1.5.5 you will need the v1.5.5 version of the jscript_framework.php file.

Due to PayPal “deleting the comma issue”, I had to upgrade from PayPal Standard to PayPal Express. After the upgrade, I received the (35) handshake failure error, which required an upgrade of Zen cart to fix.
I upgraded Zen cart from v1.5.1 to v1.5.5d and this caused “An unknown response null: :text/html; charset=utf-8: :Bad Request was received while processing an ajax call. The action you requested could not be completed” error.
Following your advice, I changed /includes/templates/MY_TEMPLATE/jscript/jscript_framework.php file by copying the one from /includes/template_default/jscript/jscript_framework.php in v155, it worked perfectly and I no longer get the pop up, Thank You.
Is that the only error that you know of from upgrading?
I am using Winchester responsive template and I still need to install Ceon Advanced Shipper 5.0.2 and Ceon URI Map 4.5.1