The shaft of a hot water circulating pump (Wilo Star Z25/2) rotates freely when the pump is not under water pressure (i.e. when the isolation valves on the suction and discharge sides are closed). As soon as water pressure is applied, the pump's motor will not rotate any more. Rotating the shaft through the plug with a screwdriver also become more difficult. Why can this happen? Can this problem be fixed?
I think your pump head isn't enough to win both circuit head and circuit pressure. If the pump was designed to operate at this kind of load, then probably it has a burnt coil.