The vehicle is a Mercedes-Benz T1 model 308D.
The engine is an old Mercedes-Benz OM601.940 4-cylinder indirect injection 58 kW (79 hp) 2299 dm3 diesel engine from year 1992. It's not a turbodiesel! - it's a classic plain naturally-aspirated diesel engine (neither turbocharged nor supercharged).
It never had a problem starting before, always started in seconds, and one day suddenly it won't start. This was April, and the previous time the vehicle was driven was December. So I try to start it in April, with the battery freshly charged, and it won't start, the starter is turning the engine like crazy, the engine makes sounds like it'd almost start, and then it doesn't. After many times trying to start, turning the engine with the starter for probably something like 5 minutes in total or more (this only works if the battery is fully freshly charged), it finally started, and once started it run fine without any issues.
After a few days I wanted to do another trip and tried to start it - same problem. This time I asked someone to pull me with a rope, we drove with me being pulled on a rope a few circles and it started.
Then I didn't use the car till July and wanted to start it a few times without success, again on one very hot day I finally succeeded after running the engine with the starter for several minutes. Then I made a test: I drove around for 1 hour so the engine became warm, then turned off the engine, and then I could start instantly without any issues. I then even turned it off again and left the engine off for half an hour - and even the it started instantly (the engine was still a bit warm). But if I left it off overnight and let it cool down completely - it won't start.
I thought its the glow plus, so I changed one, but then I thought of measuring resistance and all the remaining three had about half an ohm, so they're good as new, so didn't bother to change the rest. Also measured resistance of the lines from the glow plug relay and voltage getting there, and its all fine.
So then I thought its the starter, so I bought a new one, replaced it... and its all the same, still won't start.
What is the problem then?
What tests should I do to diagnose this?