I didn't even know the issue with the last door, which door do you mean? Maybe something was supposed to trigger by opening that door, some variable been set, which you now have skipped.

The final door of the museum, where the sarcophagus is; I found that out trying to find a solution to this

You mustn't noclip through this door, the key is in one of the two basement reseach rooms...

Originally posted by wesp5:

You mustn't noclip through this door, the key is in one of the two basement reseach rooms...

I tested this out and you're wrong.

The quest loop is actually triggered by a bug in Beckett's speech tree. If you talk to Beckett about stuff (Caine, Antediluvians, Gehenna, Thinbloods, anything other than where the sarco went) until the option to say you didn't see anything else unusual, then say so, the conversation ends just like the first time you met him. This apparently sets some variable LaCroix counts on to know quest stages.

I tested this theory by loading the save before the mission and notargeting/noclipping my way through the museum and the final door and doing the conversation both ways. Grabbing the key is an easy experience point or two but when I got the key and did the long conversation LaCroix was standing at his desk when I got back, acting like I hadn't left. You can try it yourself, and I think I'll be submitting that as a bug with the patch maintainers

Originally posted by PERSONAL BEAVIS:

If you talk to Beckett about stuff (Caine, Antediluvians, Gehenna, Thinbloods, anything other than where the sarco went) until the option to say you didn't see anything else unusual, then say so, the conversation ends just like the first time you met him.

You were right, there is one line that loops back. Found and fixed!

oh nice

Still not fixed

Can you give some details, like what exactly did he say in his last line?