Crumbling paint

I recently painted my ground floor kitchen with the correct paint.
BUT the paint is crumbling at the foot of the wall near the floor next to an external door.

I think there may be moisture rising up the wall, but I wouldnt call it rising damp. Or perhaps this is rising damp in its early stages. I don't know.

I am thinking about sanding the paint off and applying a layer of bitumen primer and then repainting. Will this work?

it sounds as though you have efflorescent salts leaching out of the masonry and crystallizing on the surface. this is common with masonry that is subject to prolonged periods of wet and dry.

it will be more beneficial if you address the problem causing the damp first.

any dripping gutters, failed dpc, leaking drains or pipes?

It may be that rainwater is getting round the frame where it should be sealed to the wall.

Is it a triangular damp patch, widest at the bottom?

Did you find crystals on the plaster when you were preparing the surface before painting?

If you have a digital camera, put up a pic.