i went back to mission area and sunk another vessel.. most likely making up for the originally requested 50k tons, and now the cross is highlighted/bold..
didn't receive any message or anything, it just auto-saved at that moment.. went back to homeport [now Bremerhaven], still received the same briefing intro, but as before the coastal lines mission wasn't selectable anymore, just this time the cross was bold as well.. i just selected the other mission of the initial 2 and will just go ahead with that.. if i finish before the due date [which will probably be the case] i'll proceed with your last suggestion until due date or whenever new missions will show up.. thx again!
