[Dominik]

In the 90s I had the most troublesome ECU controlled cars in our shop. 90% of failures are gound related.

The crank sensor is very sensitive when it comes to gapping and a pick up behind the balancer might help (axial instead of radial, if my english doesn't fail me).

ECUs typically have a compensation for voltage drop when starting with a weak battery or in winter to "convince" the management system to start irrespective. Dending on the range you could just be too low.

A welding cable for negative from the battery direct to starter cannot be a mistake (35 or 50 mm2).

Good luck!