Due to the quest "Hodder's Contact List" I have to find Hodder's Counterweight Plans in Aldenvault. According to Czelsior I have to fight Apprentive Marpin, who is mentioned in the questtext. However I can not attack or speak to him.

It could be that you first need to find all the items before you can fight him. Did you try that?

No I could not, because the other missing item drops from the bugged laughing thief.

This was a very easy fix. The quest objective requirement was incorrect. You could only fight him if you did not get the tenstion system drawings yet.
I fixed the requirement and will push it in the next update.