When did the glitch/bug occur? - After the mission (Trouble in Skyrim) in the companion's guild.
Before/under/after a mission? - After.
Minor/major glitch/bug? - Call it Major.
POI (Person of Interest)? - Farkas
What happend? - After the mission (Trouble in Skyrim), the one that lets you clear out "Moss Mother Cavern", I cleared it out. Killed the Spriggan's and returned to Farkas. Upon completing it I wanted to start yet another quest within the Companion's. I was unable to do so though. As I walked up to someone and selected the option "I'm looking for work" they only say that I am currently running an errand for Farkas. "No, I'm not! I have the quest completed and there is nothing more to it.

Extra notes? - Actually yes. Under an earlier occasion I cleared the cave already. Right before I was given the quest to clear it again. So no enemies where there. After 48 Hours of doing something else in the game (Gametime) I returned to the cave and I was able to kill all the enemies again since they respawn. Returned to Farkas and completed the quest. - But in vain, it seems.

Help?

Why not use the right forum instead?

Perhaps you could find the appropriate bug thread for your particular platform here: http://www.gamesas.com/index.php?/forum/179-skyrim-hardware-and-software-issues-subforums/
Keeping all actual bugs in the appropriate threads will help them help us.

This is a completely radiant quest, there isn't anything to spoil.

Or, or, or... Rezar could have ment the forum for hardware and software issues
http://www.gamesas.com/index.php?/forum/179-skyrim-hardware-and-software-issues-subforums/