Okay, based on what you said, I fortunately had a save right in the control room of the Foundry before the Titan fight, so I could check the two hellfire watchers behind me in the prior room, as well as the Titan + 3 up ahead after I destroyed them. In my first playthrough of this section, I had actually killed a watcher 'before' it had a chance to self-destruct. When I did this, it didn't fall over it just stood there. I couldn't target it with weapons anymore, but thought it was a visual bug as there are some here and there. I didn't at the time look at the life total. It could have been '2' or some such.
Anyhow, I went and did the fight again, and all four robots self-destructed. I immediately went to the Basilisk Gate and came back through and it 'WORKED!' So something about that Hellfire Watch robot was screwing it up as that's the only difference in before and after.
To all other folks out there, this does appear to be the glitch. Not necessarily the robots but rather an enemy that, after a battle, has >0 HP remaining. It's a pain in the rear but make sure you check all of the enemies after each fight for any that might not be zero hp but dead. As for Larian, once they pull their head out of the sand on this one, they will need to add a short bit of code that basically triggers as follows:
What is creature status? Alive - Continue actions in initiative list Dead - Change hp total to 0 (in addition to whatever else goes on behind the curtain)
I'll send this as a follow-up bug report for what it's worth. But again, check the dead. It's a lot less time consuming than losing hours and hours having to replay sections. I'll provide another update after I beat Gortash, just to confirm that doesn't compound the bug in some way. However it is already working when before it did not, so I'm confident this issue got nipped. Stay tuned.
**Update: Post Gortash Battle
Houston, we have lift off! Even after the fight, I was able to cross from Wyrm's Rock Fortress back into the Lower City. Now, there is still the massive, MASSIVE load time that occurs anytime you go from a different region back into the Lower City (whether it's Wyrm's Crossing or the Iron Throne), but if you leave it be while the little wheel is spinning, all is well. So yes, it appears for me the culprit was a Hellfire Watcher that had some HP left after it was destroyed but I didn't know it.
@Mods, please don't chastise me for not putting spoiler blanks on here. This issue is plaguing thousands of players and I wanted to get straight to the point with it.
Again, check anything you put down, animal, mineral or vegetable. Make sure it's 0 hp. If it isn't and you can't throw it in a pit or some such, pick it up and put it in your camp box and leave it there. My issue was specifically a steel watcher, but it can be absolutely anything. Until the glitch is fixed.
