If Supercharging and level 1 or 2 work fine but you get communication errors when using a CCS charger, then chances are it's related to the CCS charging station/cable plug and not due to anything internal to the EV i.e. the retrofit. There are two possibilities:
1. If the session never starts due to a communication error, try lifting and holding the CCS plug up a little during the initial start up so that not all the weight of the CCS plug and adapter is pressing against charge port. Once it's charging, you can release the CCS plug.
2. If you are getting a communication error after charging has started, then something in the CCS charging station has initiated a reset. People have reported that using a credit card on EA stations can cause this due to a station bug and the work around is to use their app to pay.
If you are still having problems, try re-installing the software for the EV.