I encountered this problem. I remember. I think it happened when you reload the game or something [not really sure how it happened]. I would suggest you replay this level non-stop and see if it happens again? If it does then try jumping off from that missing lever platform and die and let the game reload that part again. Just try around it. It should work. I remember being stuck and it did work on reload somewhere.
I just checked the issue for you mate [Lucky I still have it installed]. Stopping the game and playing later from Astrolab save is the problem. It misses the switch handle both in cutscene and lab. The only solution is to replay in continuity without switching off from previous save before Astrolab section. You will unfortunately have to replay the part where Ratash breaks off sections of palaces and makes you jump through water columns and pillars all over again including the section with traps before the Astrolab. I think it is not advisable to stop the game after reaching Astrolab and finish that part before saving further. Maybe it can be broken off after the lab starts revolving and makes you play a dicey challenge of getting across in 2nd half.
Guys don't stop the game after Astrolab checkpoint. It sets the switch on the platform off. Play the game non-stop from where Ratash breaks off walls and corridors and where you jump off water columns and pillars and then reach Astrolab through traps and a couple of minor fights. Replay from that previous point and don't stop till you activate the lever that otherwise goes missing.