Possible all right I suppose but strange that the fault only showed up when both systems on, did you note the flow gauge readings on the UFH manifold?
Is the cap on the automatic air vent on the (right side) of the flow manifold open, if not open it, release any air, close it again and test regularly for air.
Is the cap on the automatic air vent on the (right side) of the flow manifold open, if not open it, release any air, close it again and test regularly for air.