I've found the potion Vaermina's Torpor but the next script has never been triggered, i.e. I can't tell him that I got that potion. I don't have that option:
Your next stop will be the Laboratory, where Vaermina's Torpor should be stored. There are some more Invader Orcs and Vaermina Devotees that will need to be dealt with as you go along. When Vaermina's Torpor is located, Erandur instructs you to drink it and upon doing so, you become Casimir, a Vaermina Devotee during the Orc raid on the temple. You are instructed to release the Miasma to protect the Skull.
I've tried to reset the character to no avail:
It is possible that reseting his location to the player will resolve this issue (open the console and type "prid 0024280" and then "moveto player").
When I try to drink the potion I receive the message:
Quest item cannot be eat.
Similar happens when I try to remove it:
Quest item cannot be remove from inventary.
How can I solve the quest?