Posting because I haven't seen a solution for this posted. When I try to go to the roof after finding Telis' note, the elevator becomes completely non-responsive. Interacting with the house doesn't seem to trigger anything. I've played this from start to finish and I'm pretty sure you just get in the elevator and activate it and it takes you to the roof, but this doesn't happen.

What's really strange is if I interact with the elevator before the camera switches angles and shows the note, the note disappears completely. By this point, Kay'l is already used his apartment key, so it's no longer in storage or in my Sneak. I have no idea how to progress and this run has been pretty good so far. I have saved games, but no matter how far I go back, the same problem crops up.

I have the same problem. Nothing seems to fix it, including going back to earlier save. This makes the game unfinishable.

I *think* I have figured out what causes this bug.
If you recall, the first time you leave Kay'l's apartment there's a little cutscene where his wife wishes him goodbye.

If you do not revisit your apartment (and then leave normally, without that cutscene) before her emergency call, the bug occurs.

So reload an earlier save and make sure to pop back into your apartment before going to search Den's office.
I have tested this only once, mind, but I hope it helps. :D