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:
I faced the same issue and solved it by setting the settings flag Unmount System before installing a ZIP to false. This is not documented or recommended anywhere, however.
For me, it was LineageOS+MicroG image serrano3gxx. If someone else finds this to solve this error, please leave a note so we can report it accordingly.