Thanks for the replies!
I tried reloading a few times, but still the same happens.
I also checked all components (clicked on the list at the left for port/starboard propulsion) and as far as I could tell nothing was damaged.
In the end I restarted the campaign, and found another convoy of equal size and got more tonnage
Though I had this same issue again where I got shot up by a DD coming from the fog and the engines went. I think it has to be damage judging this, but for some reason my damage control station doesnt show any of the propulsion components as destroyed (though some were damaged and got repaired). Wonder if thats a possible bug? Anyone else had that?
Also a lesson leaned. Be careful when attacking in fog. Ended up ramming a ship