Yes, I've noted the same problem in another thread. The File/Save Special function was broken with Build 514.

However, there is a work-around. Instead of using 'File/Save Special', go instead to 'Audio/Render Song To Audio File (WAV, AIFF, M4A)'.

Under Format, select .m4a and then proceed to Render.

This will successfully write a .m4a file to disk. However, the file will be a lot larger than if you use the File/Save Special method. If this is a problem, you can reduce the Quality to 192Kbps or 160Kbps to make the file smaller.

I also noticed a glitch of some kind when overwriting an existing filename. To avoid this, delete existing filename first and then render new file.