I’ve copied your settings form your screenshoot and my result is pretty the same as before.
I can see the event (Hundspann) is showing on mondays but i’ve only choosed Saturday and Sunday.
In the mini Calendar the event (Hundspann) is showing every day.
Which view are u looking in?
I can confirm you that we don’t have any conflicts in our plugins installations.
Wp debug is set to “true” in wp_config.php ant dosen’t show any errors at all.
Pls, See screenshoots.