[TteFAboB]

Did the provinces went back to Belgium or are they yours? If they went back to Belgium and you're standing over Belgian territory then you need to suck on Belgian supply & oil and not your own. The controller of the province supplies the divisions inside it, his own and allied. I suppose that's why you can't set up a convoy for Ghent: it's not your port, it's a Belgian province. Maybe years and more years in Congo left them without either as they may've kept some army divisions but not the industry to supply them.

Pick the saved-game and try setting a trade agreement with Belgium. Supplies and Oil for something or nothing. Or use the "Open Negotiations" and try giving them thousands of supplies and oil at once.