[ic] Sorry, the page (process) was not found

[Davideth]

In one of our carts ( recently set up ), when clicked on [Checkout Now] 
or when doing admin/login, I am getting a "Sorry, the page (process) was 
not found" . This is actually occuring whenever the process is called on 
such as Remove an item.

It is working fine in one cart but not in the other?

Any hints as to where to look. I assume it is a permissions error and we 
have had to fix a number of those.