Koen, this isn't a leaking injector problem. The computer is confused by the lean woodgas, and is actually pouring in too much gasoline.
Once you switch off the fuel pump and run only on charcoal, the computer is getting a constant lean condition, so it will add more injector pulse-width trying to compensate. Eventually it will max out, having had no effect. Over a few miles, it will now store this value into long term fuel trim.
When you restart on gasoline the next day, the computer uses the long term value as a starting point. Oxygen sensors are offline until they warm up, so the computer works "from memory". Since this memory says " give full pulse-width to injectors and it's still not enough!" - then that's what it does. Only now with gasoline available, that's far too much fuel. The engine floods, and won't even start.
If one were to try starting on woodgas at this point, it would still be flooded from the gasoline. But repeated cranking would eventually clear it out. If you had tried the woodgas without trying gasoline first, it would have worked fine.