Hi, I had the same issue and resolved it by following the instructions from customer service inside the app. In short: you have to use Chrome, Edge, or Firefox as your default browser in the settings of /e/OS, in my case it is Firefox. You may need to close the dm-App (or force-close it) after making the settings. You can find more detailed instructions here: Once in the app, go to: My Account (Mein Konto) → Customer Service (Kundenservice) → (scroll down to) dm-App → I cannot log in… (Ich kann mich in der dm-App nicht mehr anmelden). Hope this helps you resolve the issue!
thanks for the reply! Unfortunately I already tried a different browser. I had another app, that gave me notice, that a different browser is needed, so i had this in mind.
Now, i also tried all the other suggestions, stated inside the app-help. None of them helped.
I even tried it with the advanced privacy turned off, because I thought it might help.