[Moto G7 ROMs, Kernels, Recoveries, & Other Develop] flug32:
I was having this problem as well. I have an SD Card set up as mixed (30% adoptable storage and 70% portable storage).

Now I just made complete backups of everything, and it all seemed to be successful. It does not back up the apps/app data on the Adoptable portion of the SD card, but there is a pretty easy way to work around that.

Here is what I found:

Supposedly recent versions of TWRP can read from & restore to the Adoptable Storage (SD Card formatted for internal storage).

HOWEVER - this does not seem to work if you have a "mixed" SD card where, say 20% of the SD card is devoted to adoptable storage and the remainder is portable storage

You can check what is in your Nandroid backups by opening them as .rar files. In Windows, WinRAR opens them right up. I don't see any of the adoptable storage apps backed up there, but everything else seems to be there.

Taking a Titanium Backup of the apps on the internal storage portion of the SD Card is easy and captures both the apps & data. So restoring your Nandroid backup just has an extra step: #1. Restore Nandroid. #2. Restore the remaining apps from Titanium Backup.

When you start TWRP it asks for your passcode ("Enter Password"). Here you must enter your Android passcode (the PIN you created to secure access to the device).

If you're using a pattern unlock you can convert it to numbers using this scheme.

If you don't enter you PIN code here TWRP continues and looks fine, but when you try to make a backup, it can't access the storage device and the backup fails with "createTarFork() process ended with ERROR: 255"

After you enter the PIN it goes through a process lasting several seconds that ends with "DATA partition successfully decrypted". If you don't get this message, you've got not hope of accessing the data partition for backups!

Even after that, I STILL had the "createTarFork() process ended with ERROR: 255". The cause of it this time was a few directories named things like vendor_ce/10

Looking at the log, the specific error was something like: "Error adding file '/data/vendor_ce/0' to '/data/media/TWRP/BACKUPS/8AAY0GHR3/1970-03-18--19-18-39_PQ1A181105017A1/data.f2fs.win000' Error creating backup."

Supposedly these directories are remnants of a second account on your system, or perhaps using an app like Island to set up secondary account-like system, or perhaps something else? (IMPORTANT: If you actually have dual accounts on your system, or are using an app like Island, these directories might be active app data directories, NOT remnants.)

In my case, all of the offending directories were empty. Even if not empty, the worst they *could* be is data directories for apps associated with the second account.

I deleted a few of them by hand, enough to verify that this was indeed the issue.

Then I used TWRP to install the attached .zip file RMDAD_v0.0.1.zip; installing this .zip through TWRP deletes ALL such directories. (RMDAD .zip source & more related info)

After this, the TWRP DATA backup ran successfully.

Note that I have a G7 Power, so there is a possibility the TWRP version or other issues are slightly different than with the G7.