Problem with a sdcard under Android 7.1.2 three partitions can not be mounted / used
Found the answer to my question myself here:
Solution: On devices running Android 7.0-8.1, file-based encryption (FBE) can't be used together with adoptable storage.
https://stackoverflow.com/questions/60173545/why-i-got-this-error-please-adoptable-storage-not-available-on-device-with-nat