Hooray, the updates were the solution. So far I can see, all that didn’t work, are now working.
To explain: “At the moment, please try to disable all plugins and just leave JNews Essential and Jetpack plugin. After that, please try again to access Customizer Panel and see if the problem still occurs or not.” -> I did that, that is why I asked your help. Normally I do this to check what plugin is making troubles. If I really can’t find it out, I ask help. I know you are all very busy with helping others.