C:\Music\ is the main directory
The MP4 file (m4a extension) is saved in C:\Music\Liquify
I used the "Tag -> Filename" converter %albumartist%\%album%$ifgreater(%TOTALDISCS%,1,%DISCNUMBER%-,)$num(%track%,2) %title%
The file path is now shown as \Liquify
If I want to edit a tag of that file, eg. Title in the File List View I got the attached error.
The file didn't had the %albumartist% tag set. However, the new path should be C:\Music\Liquify\ and not just \Liquify\
a) manually copy data of %artist% into %albumartist%
B) replace %albumartist% with $if2(%albumartist%,%artist%) in "Tag -> Filename" converter
I cannot see a bug when you create an illegal filename (with 2 subsequent backslashes in the middle of a path).
You should take the precautions to create only valid names with the help of filters or other appropriate measures like parameters in  as options.
I repeated your test case with a mp3 file having 3 tag-fields ...
... and got at first a good looking result ...
the filename (%_filename%) has been changed as wanted, ...
but in fact the result was bad, ...
because the filepathname (%_path%) missed the leading drive letter ...
and the given subfolder.
And also ... in the root folder of the drive, ...
where the Mp3tag application is installed, ...
there has been created a subfolder "Liquify Album" ...
containing the file "00 Liquify Title.mp3".
When I again invoked a Mp3tag action, then Mp3tag got a runtime error, ...
the same as yours, error log is attached. Mp3tagError.log (1.97 KB)
Simply said, it has been a failure situation initiated by bad input from the user.
The given formatstring for the new to be created filepathname has been evaluated to a bad string, ...
because of the missing folder name (%albumartist%).
Mp3tag should be act more robust against such failures, ...
and should reject any bad formatted formatstring, ...
when it came to apply a formatstring as filename or foldername.
Instead of ...%ALBUMARTIST%'\'%ALBUM%'\'$ifgreater(%TOTALDISCS%,1,%DISCNUMBER%'-',)$num(%TRACK%,2)''%TITLE%apply this ...$if2(%ALBUMARTIST%,$if2(%ARTIST%,'Artist unknown'))'\'$if2(%ALBUM%,'Album unknown')'\'$ifgreater(%TOTALDISCS%,1,%DISCNUMBER%'-',)$num(%TRACK%,2)''$if2(%TITLE%,'Title unknown')and even more robust ...prepend the target drive letter and folder ...'T:\TEST\MyMusic\'$if2(%ALBUMARTIST%,$if2(%ARTIST%,'Artist unknown'))'\'$if2(%ALBUM%,'Album unknown')'\'$ifgreater(%TOTALDISCS%,1,%DISCNUMBER%'-',)$num(%TRACK%,2)''$if2(%TITLE%,'Title unknown')
I don't think I should manually check everything when I want to automatically rename a bunch of files! If I should then I can do that renaming manually too. Just imagine you have 1000+ files you quickly want to move/rename so you can spot duplicates more easily.
I fully agree. Mp3tag tried to move the file to C:\Music\\Liquify\ which is an invalid path. Replacing \\ with \ for filepathnames is requested.
I also noticed a different bug. However, this causes no error but it still creates invalid filepathnames.
"Tag -> Filename" converter is set to $if2(%albumartist%,%artist%)\%album%$ifgreater(%TOTALDISCS%,1,%DISCNUMBER%-,)$num(%track%,2) %title%
%albumartist% is "MIA."
%artist% doesn't matter
%album% is "something"
If you convert Mp3tag wants to move the file to C:\Music\MIA.\something\00 title.mp3 but on Windows it's not allowed to have folder names ending with a dot. The file is actually moved to C:\Music\MIA\something\00 title.mp3 (no dot in folder name) but Mp3tag shows the path as C:\Music\MIA.\something\00 title.mp3 (with a dot in folder name). However, you still can edit the files tags, etc.
This discussion leads into a very strange direction.
This is the "Bug report" section where faulty behaviour of the program is reported. All I can see is invalid user input and the apparent expectation that this is to be rectified by the program. For advice on the program functions there is usually the "support" section.