What is the validity period shown in My Pass?

I'm not sure what you mean by "it is the next calender date”. Is there an error message in the app? If so, could you post a screenshot?

Validity period is till 26.9.2022, if I go to calender and try to scroll from august to september it just does not allow it and automatically goes byck to august

1. Which calendar is that? Where do you access it? If it's in My Pass (> Show ticket), then that is not where you activate travel days.
2. Can you look for a journey in September?
3. Can you save that journey to My Trip?
4. In My Trip, can you add that journey to My Pass (activate the yellow switch) and does that prompt you to activate a travel day? (Only do this with a future travel date! Only a future travel day can be cancelled again.)
5. After confirming to activate a new travel day, is the journey added to My Pass (listed below the barcode for that date)?

Thank you very much, if done with booking the ride first it works.

Good! The best is now to cancel your travel day because you never know what will happen. If something happens on the day of departure and you want to postpone your travel day then that won't be possible anymore. You can only cancel a travel day until midnight CET before it starts. Therefore, it's best never to activate a travel day in advance. Only do that shortly before boarding your first trains of the day.