Ask JJ to check the RateLinx Setup to see if it's an old client whose license expiration date hasn't been updated.
If that is good, then connect to the client's machine and verify the proper version of ShipLinx is installed for the machine: 32bit or 64bit.
Also make sure they aren't using an old install file.
If the client is ON-PREMISE, navigate to their local setup site (eg: http://tss-dc3/ShipLinxSetup/Admin/ServerConfig.aspx) and grab the WS Address and make sure it's set the same in the Registry.