I'm not sure this is a bug or not but, i upgraded to mirc 6.31 and haven't make any /debug since this moment.
The echo of $debug surprised me :
C:\Program Files\mIRC\[-cinpt]
/debug give me :
* Debug output on ([-cinpt])
so I've typed /debug off, and after that, all was fine.
Is this is a bug ?
Edit : not tested on a clean mirc 6.31
Edit2 : I've installed a clean mirc 6.31 that use the application data folder, and $debug works fine, odd.