Errors after installing 1.5.4- An Unknown Response null and Add to cart problem

[QuickBooksDev]

Hi

Hired a person to update ZC from 150 to 154 on Windows Server.

Most things seem to work but have seen 2 problems.

1. Error An unknown response null:text/html; charset=utf:8::Not Found was received while processing an ajax call. The action you requested could not be completed. No plugins. Happens at login, add to cart and some other places.

2. Add to Cart sometimes displays a garbage screen. Mostly dark blue, images not found. Then displays the main home page (not zencart). Currently not happening but now getting the above error instead. Yesterday did not get the above error but got the garbage screen.