Hallo @AlphaElwedritsch !
Ich antworte deine Frage in Englisch 
Ich hoffe das is OK fur dich.
Indeed, in its current state of development TWRP is useless for doing backups, as system and userdata partitions are not successfully decrypted.
TWRP without decryption ability has few use, for example:
- back-up of the Boot partition (for rooting it with Magisk)
- flashing the Ubuntu Touch system image (instructions are to erase the userdata partition so that is doesn’t have encryption)
But in my opinion the full potential of TWRP could only be with decryption, that’s why I made some trials to include it (unfortunately without success)