Mouse problem when loading saved flight plan

Lately I noticed that when I want to load a saved flight plan I have to click outside the flight planner window to have the available flight plans being displayed after I hit the "Load" button. I already uninstalled and reinstalled the client and content modules, alas,  that didn't resolve the problem. This mouse problem only occurs when attempting to load flight plans, in all other situations the mouse works as expected.

Any ideas what could cause this and how to repair this phenomenon?

Thanks,

Hans