If I use "jeden Mo, Fr um 20:00" (the example from your support documents) for my task, I get a wrong date error - if i do the same in english it's working fine...
Thank you for your report, I've reproduced this issue and have passed it to our developers. It seems to be an issue with using Monday (Montag) as "jeden Di, Fr um 20:00" seems to work. We'll look into it.
This issue has been fixed.