We know that literal brackets must be escaped with apostrophes. I am using this format:
%artist% ['['%director%']' ](%year%) %title%[ (%vocalist%)]
This mask occasionally fails, with Mp3tag showing an error message: "The filename, directory name, or volume label syntax is incorrect". Roughly one out of ten of my M4a files will fail wth that message. But if I remove the section ['['%director%']' ] then even those files will convert normally. So I am confident that the files themselves are OK. Can anyone spot what is wrong with my syntax or explain why it fails only with a few files?