I stumbled upon an error in combination with Elementor.
Containers can´t be edited when you hit the style tab in Elementor. From thereon the left editor panel is empty and it can´t be reverted. I narrowed it down by deactivating all plugins and it only occured when I activated MEC calendar again.
I reproduced it several times in different browsers and different pages.
The only solution is to delete the container and create a new one, but as soon as I hit style again the container is "doomed".
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