After Vesa from VnDyno took a look under my engine cover (thanks again!), it was clear that the transmission's cracked. It still changed gears without problems though.
So I drove home (ca. 5 kms), but after 3 kms I heard the gears singing, and when I had to stop for a red light, and tried to accelerate after, it was just ugly. You know the sound. Front axle's blocked, had to call a towing service. F***!

So anyone got a spare transmission lying around?

Thanks,
Michael