This is a bit special. You need:

1. A local music folder, e.g. C:\Music
2. This music folder also shared via network, e.g. \\mypc\Music
3. A music file in this folder, e.g. test.mp3

To reproduce:
1. Enable move files and set the target directory to \\mypc\Music
2. Load the file using the local path "C:\Music\test.mp3"
3. Save the file

Saving will result in a shutil.move(r"C:\Music\test.mp3", r"\\mypc\Music\test.mp3")

Both C:\Music\test.mp3 and \\mypc\Music\test.mp3 are different paths, but refer to the very same file. shutil.move will fail with a SameFileError

E: 09:14:17,143 util.thread.run:64: Traceback (most recent call last):
File "C:\Program Files\Python38\lib\shutil.py", line 788, in move
os.rename(src, real_dst)
OSError: [WinError 17] The system cannot move the file to a different disk drive: 'C:\\Users\\Developer\\Music\\Library
Nine Inch Nails
Ghosts I-IV
01 1 Ghosts I.mp3' -> '\\\\DEVEL-PC\\Users\\Developer\\Music\\Library
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Ghosts I-IV
01 1 Ghosts I.mp3'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File ".\picard\util\thread.py", line 60, in run
result = self.func()
File ".\picard\file.py", line 327, in _save_and_rename
new_filename = self._rename(old_filename, metadata)
File ".\picard\file.py", line 499, in _rename
move_ensure_casing(old_filename, new_filename)
File ".\picard\util\filenaming.py", line 468, in move_ensure_casing
shutil.move(source_path, target_path)
File "C:\Program Files\Python38\lib\shutil.py", line 802, in move
copy_function(src, real_dst)
File "C:\Program Files\Python38\lib\shutil.py", line 432, in copy2
copyfile(src, dst, follow_symlinks=follow_symlinks)
File "C:\Program Files\Python38\lib\shutil.py", line 241, in copyfile
raise SameFileError("

are the same file".format(src, dst))
shutil.SameFileError: 'C:\\Users\\Developer\\Music\\Library
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Ghosts I-IV
01 1 Ghosts I.mp3' and '\\\\DEVEL-PC\\Users\\Developer\\Music\\Library
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Ghosts I-IV
01 1 Ghosts I.mp3' are the same file