Hi, my name is XXXXX XXXXX I have been in the plumbing and plumbing repair field for 20 years. I will be glad to help you today. If you remove the motorized part the port valve is likely sticking. If you can free it with a little silicone spray then it will be all well. If it is not the port valve then the motorized valve will need replacing.
This clicking and sticking occurs with the motorised unit removed from the valve, so no valve sticking. So you're saying the motorised valve should complete full travel, and this funny whereby it completes it's full travel only when the heating request is cancelled? What changes inside the mech. when that happens, I'm an electrical engineer, I have a need to know when it comes to these things, I blame my father.
Inspect it while removed and you can very likely silicone spray the moving part in the motorized valve. The microswitch is likely the culprit.
OK, it needs torx drivers to get to, this at my parents, I'm travelling home today but coming back next week, must remember to bring them, I'll give it a try, if it doesn't work I'll go for a replacement, thanks for the support.