I have the same problem, but I have my own solution. I'd like to play the 30th mission of this game, but I think that it isn't possible without patch. As far as I know it's obvious that the problem has something to do with mission script. That's my opinion. Because I managed to get to 31th mission by skipping the 30th mission and that mission works perfectly.
For everybody who is interested how to get to 31th mission: Just open the directory Missions in the main game directory, look for all Mission30 files with g3c extension, back them up (not necessary), find Mission31 files with g3c extension and use them to overwrite all Mission30 g3c files. Now start up the game and you have the next mission. It's not like official help, but it helps to everybody who wants to finish this game and it's better than nothing.
To developers: Please don't judge me just because I wanted to help the players. They bought your game and it's your fault that this game can't be finished without patch. If I'd know your mission scripting, maybe I could solve it myself and repair that mission so it could be playable, but I'm not a programmer, that's your work and now, when you know where could be problem, maybe you could make a patch. We'll be waiting, all of us, we want to know what is the 30th mission about
Thanks