Okay, I have done more research. I have found that the 13mm bolt, your pic 2, is the fixing nut. That is what holds the tensioner in place after the belt is adjusted. The other pics are the adjuster which is used to place tension on the belt. See pics below. Pic 1 is the adjuster that shouldn't be turned until the 13mm fixer bolt (pic 2) is loosened.
My guess is that the 13mm bolt wasn't loosened and someone tried to turn the adjuster bolt which has caused it to strip. The only thing that can be done at this point is to remove the component and see what was damaged.
I found these pic on the net. Even my manual didn't show them.
Images (Click to enlarge)
Sunday, July 20th, 2014 AT 8:59 PM