Dakar loose frame bolt(!?)

[growweblaar]

I've just discovered a slightly worrying loose bolt on my Dakar (see pics). In the 1st pic I've removed the righthand footpeg hanger: the bolt seems to attach the bottom (black) sub-frame to the main frame. It's a long bolt that runs through the sub-frame (yellow line) and screws into a captive nut on the other side (pics 2 and 3).

When I turn the bolt I can see it spinning inside the nut on the other side, but it doesn't seem to engage the thread, so I can't tighten it.

Any ideas?

Not sure if I should be riding like this...  :-\

 

[lecap]

 

[Sharingroads]

 

[growweblaar]

 

[Dwerg]

 

[hando]

 

[growweblaar]

 

[hando]

 

[westfrogger]

 

[growweblaar]

 

[hando]

 

[growweblaar]

 

[spokie]

 

[growweblaar]