When you are about to open the last "Fragile Door" to reach the Strongroom, the conversation with Mel breaks and it's impossible to advance.

That also breaks the ability to talk with any other NPC, so the game entirely is rendered useless...

I have tried all non-conventional hacks found on the web, like killing him, moving him, reseting him, TAI'ing him, nothing works.

Even if I advance with TCL, the following conversations does not happen.

Please patch this!

I was finally able to complete the mission.

I had to kill Mel - after he got hostile, I used "stopcombat" on him, and was able to finish the conversation and advance.