Na bj 97 no longer starts… immobilizer???

Moin municipality Something is always…. Wanted to start my mx Na Bj.97 tonight….but after a short jump, he immediately went out again and jumps again since then. Starter turns, but nothing is happening. He has a factory-side immobilizer in the key. Can this be defective? How does it even work? I parked the car three days ago and before it ran without problems, had power and a round idle. Now as I said, except orgeln Nothing. It’s under a carport, so it’s also relatively dry. Does anyone have a tip? Greetings Romanus