I have had a similar problem on my 54. I have not yet got to the bottom of it but I believe it is the top and bottom sensors that are not actually sensors. They are simply push button switches. When the frame is lowered it clamps down on the bottom switch. There are two cables on a toggle, and ditto at the top. This then sounds the alarm and stops the motor running.
I found that if I pressed them a few times by hand then it worked fine. So I suspect there is a poor/ intermittent connection within the button/toggle.
I reiterate I am not 100% sure about this. It is what I think
If you try this and it works then all you need is to order a new pair and a spare pair because I think this is a common fault.
I would love to know, as I have left my boat out of the water in Antigua whilst I am in the UK, and would order the parts if that is indeed the problem.
Amelia (54 #019)