For me, it looks like a hydraulic problem. This is the first thing which comes to my mind. Mine had the same problem. The transmission will disengage if you use the left pedal. The actuator that performs it is on the tranny, and it has a strange balancing beam configuration that allows it to work in both directions. It took me about an hour to find out how it should have been altered and make the necessary changes. With a handbook, it would have been a lot easier. I decided to gift it to my brother after repairing it, and bought a new one from https://www.machinerydealer.co.uk/, now there are no problems at all.