I also stumpled upon the createTarFork() ERROR 255. These are the last lines of recovery.log:
I:addFile '/data/vendor_ce/0' including root: 1
==> set selinux context: u:object_r:vendor_data_file:s0
failed to lookup tar policy for '/data/vendor_ce/0' - 'b375d30057e757ba'
I:Error adding file '/data/vendor_ce/0' to '/external_sd/TWRP/BACKUPS/A209GQ1S0202/1970-05-19--13-05-18_89012A010520191217/data.ext4.win001'
Error creating backup.
I:ERROR tarList for thread ID 0
Error creating backup.
I:InfoManager saving '/external_sd/TWRP/BACKUPS/A209GQ1S0202/1970-05-19--13-05-18_89012A010520191217/data.info'
createTarFork() process ended with ERROR: 255
Backup Failed. Cleaning Backup Folder.
I’m on an unlocked FP3, Build No. 8901.2.A.0105.20191217. I have removed many Google Apps as of here. I have played around with the multi user feature, but have removed all additional users. When I started TWRP using fastboot boot <twrp.img>, I mounted system read-only.
What do you make of it?
PS.: I am going to PM you my recovery.log