Volker, thanks for the screenshot. I can see you’re not using a default WordPress theme, like Twenty Fifteen.
Could you try temporarily activating the default Twenty Fifteen theme and seeing if the issue persists? (Please try with and without the checkbox checked to see if you notice anything that may be helpful in troubleshooting.)
If disabling the theme fixes it, we have narrowed the issue down to a theme conflict. Do you have any theme overrides for the Events Calendar? If so could you try disabling them by renaming your [themename]/tribe-events/ folder to ‘tribe-events-bak’. Did that fix it?
If the issue persists in the default Twenty Fifteen theme, then we have a different set of debugging steps. Please keep the Twenty Fifteen theme enabled, and also disable any plugins other than the ones from Modern Tribe to see if that fixes it. If it does, please try re-enabling the plugins one at a time until the issue resurfaces. When it does resurface, can you let me know which plugin caused that to happen?