Today i wanted to reload my credits. I had to acknowledge to my regret that a payment on my preferred bank transfer is no longer possible. I would like to know why. Was this method to of payment too uncertain, or what is the reason?

You may write a ticket to the customer service. Click on the "i" with a circle around at the top right of the window to find the ticket facility.

I wrote a message to the customer service. I hope that they will offer the direct banking again.

I think that has something to do with the new EU Payment Services Directive (PSD2), which has been in operation since 15 September 2019. After that also direct debit should be possible only by 2 factor authentication.
I think there is still a waiting period, because not really the practice has been clarified.