Rasmus,
I personally think that disconnecting the battery had nothing to do with solving your problem.
It’s not like modern cars, the ECU in our cars has no memory, no adaptive features and no reset.
When you turn the ignition off all power to the ECU is cut off.
I think it was the time factor, something cooled down and started working again.
Unfortunately, first and outmost culprit is the ECU.
Had the same problem but opposite, my ECU would work only when warmed up, impossible to start but when it did it run absolutely perfect.
Ignition module or the coax wire could also cause such problems as it communicates with the ECU via the coax wire.