There is a bug with finding the cure ingredients and moving them in your inventory before talking to the alchemist. That triggers a quest log update prematurely, and prevents the cut scene at the rock covering the mushroom cave. There are a couple ways to work around this problem;
[*] Complete all other quests and save the game.
[*] If you have a save before finding the alchemist's ingredients and moving them, copy the 2 story.* files from that save into your latest save, otherwise you can use
these files from a save at the beginning of the imp village.
[*] Talk to Altoflix in the north of the village (if required), then the chieftain's son to get the quest to find the alchemist, talk to the alchemist once (only) and head to the cave.
[*] Continue with the game.
or bypass the entrance;
Teleporting anywhere (via hex editor)