Impossible to load gas

Hi Two Opels, a 1989 Vectra and a 2004 Combo. Both have LPG conversion kits installed. The refueling nozzle is Italian, so I need an adapter to refuel in Spain. I have one, made God knows where, and I have no problems refueling the Vectra, but not the Combo. While it's safe to assume my adapter is faulty, the same thing happens with adapters from the gas station itself, which are supposed to be standard. It pumps a little, and after a while, visible vapors start to appear, followed by liquid gas leaking out. The only difference between the two kits is that the Vectra's has an evaporator, while the Combo's has injectors.
I asked at several workshops, and they all tried to sell me a new nozzle and adapter, telling me that the nozzle was faulty. But that doesn't convince me, because if it were true, it would cause problems, especially in other countries where the same nozzle system is used and the spray guns are designed for that system.
Well, if anyone has any idea what's going on or can tell me where I can find information on how to solve this problem, I would be grateful.

Thank you.