That's...odd. Never had any person report an issue with this in literally several years now.
Here's the exact code that is the only way to prompt that message:
if (!GetLocalInt(GetModule(), "boss1") || !GetLocalInt(GetModule(), "boss2") || !GetLocalInt(GetModule(), "boss3"))
{
FloatingTextStringOnCreature("The celestials need your aid on the southern front first.", oPC, FALSE);
return;
}
So...it literally just checks if the three southern bosses are dead -- doesn't even force you to "check-in" with an NPC or anything. When you tried to go east was there another event which had Succubi as some of the enemies? If not, then that trigger doesn't think you've killed all three bosses to the south either. Looking at the code the bosses it's also very straightforward -- just sets those values to 1 when the corresponding boss dies. Nothing fancy.
Did you get a journal entry for each boss when you defeated them? If you don't have one for each then that would be a problem as the journal entry is tied to the setting of the variable indicating the boss is dead.