Year about var 4, a bug in the game or what is it?

Help me to understand! I play in the year of var 4 according to the plot of the doshol until the moment where you need to return with the light of alfheim. The passage to the mountain barely killed like a level 6 velve, then you need to insert Crystal so that a bridge appears, I can't do this because the plague fires that the witch enchanted, the last one next to the crystal is also enchanted, but the poison from it diverges and nothing can be done? That this is a bug in the game or is there something wrong with the disc? Who knows, tell me?