E1001: Failed to update System image latest Onplus 5T and other

I downloaded the latest LineageOS for my latest Device, from Dumplings SF. firmware, recovery, vendor files. What I get all the time is that error message "E1001: Failed to update system image" and that he can't mount the vendor partition. Any ideas ? PS: I can't even flash back to latest Oneplus ROM (or even older roms).... Update: Meanwhile I use TWRP from "cheeseburgerdumplings" for 8.1 on what lineageOS and the "/vendor" partition is no more tried to be mounted. What I figure out as well, is that if I go with adbs command to know if the filesystem is encrypted or not: adb shell getprop ro.crypto.state tells me "encrypted" Now the big question, which filesystems are encrypted, all ? And which partition or FS does not have to be encrpyted? Here is the "recovery.log": https://pastebin.com/raw/tKMStdSF Screenshot: