Cannot set a weekly repetition for an event

I added an event to a calendar. After that I edited the event and tried to add a weekly repetition, ending on 26.10.2015 (selected the date from the date picker).

After clicking OK I get an error telling me that the date format is wrong (showing the exact same date format as entered).

(German localization)

After selecting the date in the date picker, the edit box flashes the message "Formatierung" ("formatting" or possibly "format").
Entering date manually does not help.