Hello

Today I was driving my 98 A4 1.8 Turbo (FWD Auto) to the store, when I came out 5 minutes later I started the car, drove off the parking and 5 meters on public road the car died. It felt almost like if it was out of gasoline (which it is not)
I tried to start it again in "panic" to avoid blocking traffic but it does not crank at all. It's almost like if its waterlocked or a piston is stuck, only a small tick noise when the starter motor tries to kick in (relay sound)

A bit later tonight i opened the oil filler cap and underneat it there was a bit of white "foam" sitting on the underside of it, could be water in oil/head gasket?

No water is missing from the cooling container

the last days it has struggled a bit at running without my foot on the throttle, dropping rpms and getting back to normal rpm. like if you cut the air supply or something a couple times a minute

I'm new to this forum and decided to register here as this seemed to be a nice forum ( i entered a few from googling audi forum, this one looked active and clean)
thanks in advice