Yeah, of course, you're right. This morning tried to solve the problem again.
I'm sure ground is ok. I think starter is not getting enough volts. After doing some test noticed a thing:
Connected multimeter to battery: 13.5 volts, everything off
Turn key to "ignition on": voltage at battery drops to 11 volts.
IMO to low for starter.
Disconnected voltageregulator: voltage back to 13.5 volts.
Disconnected alternator: voltage also comes back to 13.5 volts.
Is this normal??

Now I suspect the alternator because:

Earlier I had to drill out one of the mountingbolts of the alternator because it had snapped of. While doing this I hit the windings of alternator so there might be a short cicuit in the alt now? Must tell you the startingproblems started after I worked on the alt....