Cats die because fuel control doesn't (control). Unburned hydrocarbons are the death to cats. This means all forms of incomplete combustion.
But, figuring that vehicle to be a converted car, I would highly suspect your feedback control. I would monitor its activity as the first choice even if it was a US model. Most of the Euro M110 engines had rather poorly constructed hang-on lambda control.
Without functioning lambda (closed-loop) control a three-way cat TWC will eat itself up rather quickly.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician