mount failure with code 255?

[chr4ss12]

i am getting this when trying to run tetherback on galaxy A5 520F:

tetherback v0.9.1
Found ADB version 1.0.41
Using default transfer method: adb exec-out pipe (--exec-out)
Device reports kernel 3.18.91-g05cf350e
Device reports TWRP version 3.7.0_9-0
Reading partition map for mmcblk0 (25 partitions)...
  partition map: 100%                                                                                                                                                
Reading partition map for mmcblk0boot0 (0 partitions)...
  partition map: 100%                                                                                                                                                
Reading partition map for mmcblk0boot1 (0 partitions)...
  partition map: 100%                                                                                                                                                
Reading partition map for mmcblk0rpmb (0 partitions)...
  partition map: 100%                                                                                                                                                
Saving backup images in ./twrp-backup-2023-10-30--18-08-48/ ...
Saving partition boot (mmcblk0p10), 32 MiB uncompressed...
  boot.emmc.win: 100%   2.1 MiB/s  10.1 MiB                                                                                                                          
Saving tarball of mmcblk0p19 (mounted at /system), 4848 MiB uncompressed...
  system.ext4.win: 100%   2.6 MiB/s   2.0 GiB                                                                                                                        
Saving tarball of mmcblk0p25 (mounted at /data), 24468 MiB uncompressed...
Traceback (most recent call last):
  File "/opt/homebrew/bin/tetherback", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/tetherback/tetherback.py", line 359, in main
    backup_partition(adb, partmap[standard], bp, args.transport, backupdir, args.verify)
  File "/opt/homebrew/lib/python3.11/site-packages/tetherback/tetherback.py", line 247, in backup_partition
    fstype = really_mount(adb, '/dev/block/'+pi.devname, pi.mountpoint)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/tetherback/adb_stuff.py", line 26, in really_mount
    if adb.check_output(('shell',f'mount -o {opts} {dev} {node} 2>/dev/null && echo ok')).strip():
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/tetherback/adb_wrapper.py", line 35, in check_output
    return sp.check_output(self.adbcmd(adbargs), universal_newlines=un, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.11/3.11.5/Frameworks/Python.framework/Versions/3.11/lib/python3.11/subprocess.py", line 466, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/Cellar/python@3.11/3.11.5/Frameworks/Python.framework/Versions/3.11/lib/python3.11/subprocess.py", line 571, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '('adb', '-d', 'shell', 'mount -o ro /dev/block/mmcblk0p25 /data 2>/dev/null && echo ok')' returned non-zero exit status 255.```