I do not have a toast install on my computer here or home. I am sure the disk is around somewhere so the Mount it will have to wait. I did uncheck and recheck in the toast preferences and it made no difference.
In toast it self, using the mount when in the copy section does not work, gives the error above. Using Utility>Mount Disc Image give a Mac OS Error 2 and the error above. So it looks like the only way to view toast image files is to change it to .dmg as that works. In fact, there is an instant open, no window showing something is mounting.
So it seems that toast 8 works pretty much, as long as it does not access some power PC app to assist it as what seems to happen above.