This time I managed to 'cause' a similar bug due some mouse lag. Some of the assassins in the Dxun encampment weren't removed because I improperly left the map (normally you are turned around, but due to mouse lag I sort of jumped straight into the jungle map). Upon returning to Dxun to deal with the palace, the NPC that tells you there is a message from Tobin never gets to complete his dialogue because he runs off for battle. Talking to him again puts me way at the end of the dalogue tree and I don't get to choose a party to assault the Sith temple.
Instead at the Temple I only get Atton, who is invisible. After some saving/reloading/dying I managed to make him visible again. But still, only Atton is in my party so I cheated and made him invincible, just to get through the Temple. Now, after clearing out the inner area, a mandelorian appears to take me back to my main character's efforts on Onderon. At THIS point I appear to run into the same bug ppeterok1 helped to clarify, as I don't have enough party members and the screen goes black & crashes right after the dialogue.
Interesting note: I tried to use KSE to add two more party members to my save, but it forbids me from adding the 3rd party member. Probably because it mistakenly thinks the Exile is in it. Is there a way besides KSE to add the last member to my party?