Lost the panel

It’s likely a javascript conflict with another plugin, please follow our troubleshooting FAQ in order to locate the source of the problem. In particular check your browser’s javascript console for errors.

I deactivated all plugins an reactivated them one by one. The conflict is obiously with Jetpack. If I deactivate that TinyMCE works fine.

This is a known issue (depending on Jetpack). Please check the following threads:
https://www.ads-software.com/support/topic/bug-with-jetpack-usage/
https://www.ads-software.com/support/topic/5-2-breaks-text-and-tinymce-widgets/