Android - TWRP createTarFork() error 255 when backing up data
For me, the error was related to the I/O or file corruption issue.
To find what has happened, browse to the backup location (usually
/external_sd/TWRP/BACKUPS/backupfolder) and find the
recovery.log file. Open the file, and at the bottom lines, read what folder caused backup failed.
Delete the corrupted folders using TWRP File Manager, delete the old backup folder, and try to do a backup again. Consider that this may occur for different folders, so you need to read the log file every time.
Currently, TWRP doesn't support multiple user profiles or what OnePlus calls Parallel Apps.
This XDA post has a work-around.