an update, and likely (fingers crossed) the last post on this issue for another 6 months or so. when we noticed the paint bubbling in our wall, we had immediate panic.
6+1 (six called by us -a general contractor, 3 masons, one mold remediation specialist and a roofer- and one by our neighbor – an hvac contractor) contractor visits later, it turned out the water damage on our wall was coming from a condensation that build up because the furnace at the house next door had no outlet, unbeknownst to the owner.
once we figured out what was happening, we proceeded to take off all the damaged paint.
ps- probably a wise move to put on the breathing mask due to all the lead paint (which sergey would not let me to touch)
it looks like we avoided the worst scenario (knock on wood). Since there are no cracks in the plaster we just have to wait for the wall to dry out – about 6 months of ugly – before sanding out the edges and repainting.
pps – that brown color is old glue, likely the wall was wallpaperd at some point?