Golf 4 AHF won't start - Troubleshooting

Hello,
My Golf 4 with the AHF engine, manufactured in 4/98, won't start. The starter motor is turning. I had the same problem two weeks ago, but the engine eventually started after a while. Today, we tried to tow the car (in second gear, up to 25 km/h), but it still won't start. I disconnected the return line from the diesel filter (I couldn't disconnect the supply line due to a lack of tools), and almost nothing comes out when I try to start it. I heard somewhere that the AHF engine doesn't have a fuel pump in the tank – you can't even hear the pump briefly activating. There are no error codes in the diagnostic system.
What else could I check?
Thank you for your help.

Can the immobilizer be deactivated?
Is the battery still okay (minimum speed)?
Opening the fuel line beforehand won't help; there's no pump in the tank. However, doing so could introduce air into the fuel injection pump.
You can disconnect a fuel line leading to an injector to see if diesel is flowing during startup.

The battery is one year old, and initially, only the starter motor was suspected. I drove a short distance using only the starter motor , and that worked.

update:
I've 'checked' all the connectors at the pump - everything looks normal - no corrosion or anything like that. We opened one of the four pump outlets, and beautifully, diesel is spraying through the engine compartment - and it started. So, we reconnected the line, and it's running. Of course, it's unsatisfying not to know what's causing it. There are air bubbles visible in the diesel supply line after the filter - but could that be why it won't start at all? Could it be that the pump was completely full of air, and opening the outlet and starting the engine forced the air out, or got enough diesel back into the pump? After a test drive, you can still see air bubbles, but really very few.
I wouldn't necessarily overreact to the white smoke coming from the exhaust after starting the engine and during the initial test run, especially considering the repeated attempts to start it, right?

How can I disable the immobilizer that is 'malfunctioning' intermittently? Should this be stored in the error memory or something like that?

The "air in the line" theory is the most likely explanation here. By disconnecting the hose, you've bled the air from the pump. You could also try repeatedly starting the car until the doctor arrives, but that will take forever and drain the battery.

The white smoke is likely unburned diesel fuel. I wouldn't overreact to it, as long as it disappears during the next (successful) start attempts.

If the immobilizer is the problem, you'll mainly notice it because the engine will start and then shut off after about a second. But it will definitely start briefly at first.

It's not necessarily a bad thing that it runs after bleeding, now you know what the problem was. So, find the leak and seal it. I would start by checking the filter...

Best regards,

Is the fuel line from the tank to the filter a continuous piece? Is it a major undertaking to replace it?