I've been having the same trouble. I just restored my XT1955-5 to stock RETAIL. Then installed TWRP and Magisk. I set my phone up as I like it, and want to back it up - everything I can possibly backup - boot, data, system, system image, efs, persist1, persist2, fsg, vendor, vendor image, dsp, and boot logo.
Before I tried to backup anything, I made sure that I mounted all of the partitions in TWRP.
The first few times I tried to back them all up at once, I got the error 255. So I started backing up the partitions one by one. The only partition that would NOT backup was the data partition. I still get the error 255 on the data partition backup. But all the others worked fine. I even went back and selected all the partitions except the data partition, and it backed up in one large backup just fine. So I don't know what the deal is with the data partition, but I can grab it with adb pull, so I guess I'm good to go.
I even booted back into the system and turned off the screen lock PIN - changed it to none. But I still couldn't backup the data partition via TWRP.
This should work for you from adb to backup your data partition:
Code:
adb push mmcblk0p64-data.img /dev/block/mmcblk0p64