The error "Could not communicate order status change to PAP, Your API file may not match your PAP installation." is caused by different versions of your API file and the version of your PAP4 installation. This happens when you use API file on another server than your PAP4 server is. The error probably appeared after you have updated PAP4 application but forgot to update the API file on the other server.
To solve the problem, copy the API file from the actual installation and overwrite the API file at your another server.