@Franz-Joli ,
The original Compare Plugin is outdated, unsupported, and might not be 100% compatible with Notepad++ anymore. Only use ComparePlus, unless you have a reason why. (ComparePlus was a significant rewrite of Compare Plugin, with lots of new features and improvements.)
You also mentioned “uninstalling the plugins and reinstalling them” – that sounds like you have both plugins installed at the same time. I highly doubt that it’s a good idea to have both Compare and ComparePlus installed on the same instance.
As far as I can tell, neither Compare nor ComparePlus has a “cancel” action. They have a “Clear Active Compare” action. Are you saying that you hit that “Clear Active Compare” button while it’s still working on making the comparison, to stop it in the middle? Or do you just mean that you are clearing the comparison once it’s done?
I have no idea what might happen if you Clear Active Compare before it’s finished doing the compare – it might be that it’s not clearing all the memory or some such, depending on the timing.
But with normal usage (telling it to compare, looking at the results, clearing the active compare), I have never gotten to the point that the plugin just hangs indefinitely. If you have a pair of files (with no private information) that you can get to cause the hang reliably, and can explain, in detail, the procedure that you use to cause the freeze, then I would recommend that you create an issue at the plugin’s GitHub repo so that you can let @pnedev know about the problem.