If you get this error there probably is a file with this filename you want to create already in the target directory.
This includes files that you are creating if you have marked several files.
I understand that. So to rule that out, I changed the name of the artist and tried again. Same message. So for example, I was trying to edit metadata for AC/DC. Got that message, then changed the artist metadata to just AC to see what would happen. It wouldn't rename the files with AC as the artist name instead of ACDC.
I see that this was an issue with an earlier version of mp3tag. I'm curious if it has cropped up again.
as the / is an invalid character for filenames (and folders), it would be better to embed the tag-filename mask in a $validate() statement.
Alternatively, you can replace the / with ⁄ (which looks like the / but has the code U+2044 instead of U+002f).
Perhaps the error message is a little misleading as really the file cannot be created at all with that filename, not because there is an existing one.
Because you were renaming when the error occured, it means that the tag values in the group of files you were renaming resulted in two files having the same file name.
Best way to avoid it is to include the track number in your file naming pattern so that filenames are always going to be unique, always check the track column value is greater than 0.