[M491Cabriolet]

I had a similar problem once and it drove me crazy. I did everything you are doing and all looked ok. Still no cranking from the starter. I tested the solenoid and found the starter to be fine. Finally, by pure luck, I randomly touched the 14 pin(?) connector near the cdi box and, voila, magic! So I unplugged it, blew it a kiss, plugged it back in and never heard from that gremlin again. I think it must have been corrosion in the plug. I hope your issue is as simple and cheap as mine was.