I made some tests with the same file: impact.ttf.
Version 6.12:
/font -z 12 c:\Test\impact.ttf DONT crashed.
/font -z 12 c:\mIRC\impact.ttf DONT crashed.
Version 6.15:
/font -z 12 c:\Test\impact.ttf DONT crashed.
/font -z 12 c:\mIRC\impact.ttf CRASHED.
Version 6.16:
/font -z 12 c:\Test\impact.ttf DONT crashed.
/fonte -z 12 c:\mIRC\impact.ttf CRASHED.
I think if the font file is inside \mIRC directory crashes. And before 6.15 version didnt crash.