Toad Config maybe. I don't think Oracle version and client would cause this...esp since output file is getting created.
Does it happen for simpler scripts? (what if you just run the sql that doesn't get a grid all by itself?....or the last 10 instead of all 31?)
If you send me your user files folder (where all your Toad settings are) I'll try again. Instructions on how to do that as well as my email address are here.
-John