I have no experience with OTG backup, sorry. But error 255 means low storage space, so check how much space is left there. If the card is empty and still doesn't have enough space to back everything up, try backing up fewer files, copying them to pc, then erasing the card and backing up other files, until you have backed up everything.
If you back up by excluding only data, it will finish without error.
Perhaps there is something wrong with the data?
I formatted the 32gb sd card and tried backing up only the data.
But the result is the same with an error.
When I backed up the data with adb, it was 20.8gb.
With Havoc-OS 4.6, I was able to back up without any problems.
I think it's probably a problem with LOS 19.1.