Solid floors are prone to condensation because of heat loss close to the ground. The impermeable surface acts as an amplifier, resulting in water under the skirting board. It is also common for plasters to allow the plaster to touch the solid floor. plaster is absorbent. The result is often misdiagnosed as rusing damp. The simpest solution is 1) improve ventilation, 2) seal up at the base of the skriting board, 3) cut the plaster back at the base of the wall by 10 – 20mm back to brick (chisel, grinder or better still Oscillating Multi Tool, such as Bosch), 3) Run a bead ~10mm of a absorption reducing Thixotropic cream such as DryZone by Safeguard (Amazon etc), along the exposed brick. Replace the skirting board.