Wessex Lift Boy GB1 Mk III starts then reverses?

I have 2 wooden up & over garage doors operated by 2 Wessex Lift Boys. Yesterday, one began to start to open, move 6 inches then reverse back to where it started.
I have tried powering off for a couple of hours, have removed the cover and blown out the cobwebs. I have also cleaned the shaft and shaft sensor as advised in another thread. Nothing made any difference.
I then tried unplugging the 5-wire loom and plug from the PCB. Everything then worked normally.
I am reluctant to just leave it unplugged as I do not know what effect that has on the total system.
Is there anyone around who still understands the Lift Boy electronics and can advise what might be the underlying problem and what is the effect of leaving that plug and loom disconnected?
Many thanks