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 PAP installation. This happens when you use the API file on a different server than your PAP server. The error probably appeared after you updated the PAP 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 on your other server.