2003 Z4 E85 2.5i: won't start after a quick errand, but cranks fine. Key cycle and it fires straight away

[detch]

* Start a car
* Drive it, get it up to the temperature
* It drives fine, has power, no misfires etc.
* Stop for 10-20 minutes
* Try to start the car
* It cranks forever
* Turn the key all the way back to off, then crank again
* It starts right away

It's not a bulletproof scenario, although I noticed it happens most often.
Only stored code is P1511 (DISA circuit), which as far as I know can't cause a no-start.
Once or twice it also struggled to start while being cold.

Any clues how to diagnose it further?

 

[bigwinn]

Difficult starting once warm is usually linked back to crank position sensor or a similar engine position sensor

I’d start with cps

 

[Robtp10]

I had teh same on mine a while back, it turned out to be the Crank Position Sensor