Engine control unit disconnected and reconnected, vehicle malfunctioning.

Okay, here's the translation:

'I'm posting this issue here because it's not directly related to CAN.'

I disconnected the engine control unit (Golf 5) by unplugging the two large connectors while the battery was disconnected, and then plugged it back in. Subsequently, the vehicle started again and could even be started.

After an hour with the battery connected, absolutely nothing worked. When you turn the ignition key, the instrument panel just flashes erratically, while a distinct clicking sound comes from the engine compartment. After a short period of time, the vehicle will stop working again.

If you disconnect the engine control unit, you can at least achieve a constant positive voltage and tap into the remaining communication buses.

Does anyone have any idea what happened here? Something to do with the immobilizer? Why could a problem occur even though the battery was disconnected when the engine control unit was removed?

Newer vehicles often react negatively to a disconnected battery.
It's quite possible that you'll need to use VAG-COM to clear all the error codes that are present in the system in order for the car to function properly again.

SGM

For issues like that, the best thing to do is always to read the error codes from the vehicle's computer.

Anything else would be speculation.

Okay, as Max already said, delete everything!