My environment (although probably not distribution specific) is Ubuntu Focal 20.04 using default packages.  After a recent upgrade from 4.11.x to 4.13.14 the domain backup command has started failing with this exception:

running backup on dirs: /var/lib/samba/private /var/lib/samba /etc/samba                                                                    
ERROR(<class 'FileNotFoundError'>): uncaught exception - [Errno 2] No such file or directory: '/var/lib/samba/dfs/netlogon'                 
  File "/usr/lib/python3/dist-packages/samba/netcmd/__init__.py", line 186, in _run                                                         
    return self.run(*args, **kwargs)                                                                                                        
  File "/usr/lib/python3/dist-packages/samba/netcmd/domain_backup.py", line 1105, in run                                                    
    if any(os.path.samefile(full_path, file) for file in all_files):                                                                        
  File "/usr/lib/python3/dist-packages/samba/netcmd/domain_backup.py", line 1105, in <genexpr>                                              
    if any(os.path.samefile(full_path, file) for file in all_files):                                                                        
  File "/usr/lib/python3.8/genericpath.py", line 100, in samefile                                                                           
    s1 = os.stat(f1)                                                                                                                        

$ apt-cache policy samba
samba:
  Installed: 2:4.13.14+dfsg-0ubuntu0.20.04.1
  Candidate: 2:4.13.14+dfsg-0ubuntu0.20.04.1
  Version table:
 *** 2:4.13.14+dfsg-0ubuntu0.20.04.1 500
        500 http://gb.archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages
        100 /var/lib/dpkg/status
     2:4.11.6+dfsg-0ubuntu1 500
        500 http://gb.archive.ubuntu.com/ubuntu focal/main amd64 Packages

The backup is executed as:

/usr/bin/samba-tool domain backup offline --targetdir="${BACKDIR}"

I suspect that the cause of the issue is having a [dfs] share under /var/lib/samba which is included in the backup.  The directory contains dangling symbolic links of the form:

lrwxrwxrwx 1 root root 39 Mar 26  2021 sysvol -> msdfs:dc0\sysvol,dc1\sysvol