I am not familiar with the V12 cars, but as Arthur and others indicate, this seems to be a foreign fluid ingestion.

It sound like the engine is receiving a foreign fluid through a vac line somewhere. Whatever it is it's been happening for a long time because it plugged up a catalytic converter. Since only one converter has been replaced, I would try to isolate every vacuum line that draws vacuum on that bank.

Once you've found the problem you will probably have to replace the cats.

Good luck and keep us posted. This one will require patience and dilligence. I do believe that the source of the problem will be something relatively simple and inexpensive once you find it. After the fluid source is found and fixed, write down all your codes and clear them then go from there. You may have a few other gremlins to chase out.

Good luck,