A funny thing...I tried your suggestions, had no luck, and figured I would have to go back to the beginning of Act IV and try again (didn't want to bother you with save files for a problem no one else seems to have had). So I took a break and went to play the campaign from HOMM1. Then I returned to tackle MM IX and, lo and behold, the door opened! Do ya think maybe they talk to each other?
Anyway, I finished the game, thanks in large part to the help found here. I'm going to play it again using what I learned this time through.

Thanks again,