Most two way switching problems are caused by some interference on the two white control wires. Some basic checks can be done. Remove all the two way switching connections and check the dimming operation. Next connect a remote bell-press switch with completely separate wires (hence not in the conduit or existing harness – run loose wires on the floor for a distance to the other switch). Check operation, if the dimmer operates correctly, inspect the wire harness for possible short or incorrect wiring

Make sure that there are no mains live, neutral or earth connected to the two white wires or any bell-press switch connected to the white wires.