The wall is probably absorbing water and the plaster is falling off. What to do?

Dear construction professionals,

Around our house is a very old brick wall that has been plastered and painted. On top of it, Schrellevis boards have been laid. After two winters, the plaster is peeling off in many places (see photos). We suspect that the very old wall is drawing water from below. But we are not entirely sure. What do you think could be the reason here? And almost more importantly, what can we do to save the wall?

Thank you very much in advance! I am always amazed at how wonderfully people help here...

Best regards Daniel

It was probably only a bituminous coating applied before the first row of stones back then. I believe nowadays such walls are built on a barrier membrane, similar to house construction starting from the ground slab. I suspect it would be more economical in the long run to rebuild the wall. Messing around with renovation plaster, etc., will not bring joy in the long term. Since the wall is not particularly high, I do not see a cost explosion with a new construction. But of course, that always depends on one's perspective. I would not hesitate long and see it as an investment in peace of mind.