Bug in "Why can't we be friends?"

[Eisendrache]

The first thing, that bugged me was, that I couldn't reach Lem, cause his door was locked. So I did Cookes quest with killing the geckos. Easy. While doing that, I crossed the Microcline Rock. With some Status checks, I had the option to blow up the cave and the vault. I ignored it and killed all geckos. Then I went to the difficult door and terminal, to reach the overseers office and Lem. There he gave me the quest, to destroy the Microcline Rock. After telling Cooke of my sucess, I went back in the cave to just destroy the cave (have enough C4).
But I still get only the option blow up cave and vault. Now what?