2.mIRC now uses its own custom ini routines to ensure more reliable ini file handling. They should work almost identically to the standard Windows ini routines.
I don't know how but it seems Windows was able to handle [ ] correctly, even though you would see the same character used for both, and it seems that the new routine is unable to handle them, for me it's a bug.
edit: well in fact it's not a problem nor a bug, you don't have to bother about how it's handled as long as it's working correctly, and it is, 7.X use it's own routine but handle this case fine.
