Normally the OPART300 editor can load the regular OPART scenarios without problem. But occasionally one will run into one that it can't load and you have to switch to the smaller executable to load it.
I finally stumbled across the cause. If you save a scenario from the OPART300 editor using the "F12" key, then that scenario can't be reloaded back into OPART300. To fix this, simply load the scenario into the regular OPART editor and save it from there. Then OPART300 can load it again.