This bug concerns the "Find the lyra of Apollo" quest. Don't start this with special keys in your inventory, since you're likely to lose them all. This bug has already been reported here.

One probable workaround would be to make sure no special key is in the inventory before starting this quest. I discovered this the hard way. Now, if I want to visit the barracks or the symposium of the sun, and probably a few other places, I will have to restart a new game...

Hope this helps !

Thanks for the heads-up!