Hey Dirk,
Thanks for reaching out to us!
This is a bug which we recently discovered as well. A fix is already under development and should be released still this month if all goes according to plan.
To get it working until the fix arrives here is what you can do:
Open the wp-content\plugins\events-calendar-pro\lang\tribe-events-calendar-pro-xx_XX.po translation file with an appropriate app (e.g. Poedit) and change the translation of the English source string ‘all’ to ‘all’ (or remove it) instead of having it translated to your language. Save and upload.
Again sorry for this, a fix is on the way.
Let me know if the above fixes it for you.
Thanks