Ground wire arcing in breaker box
The starting point here is that your building does not have a Grounding Electrode System. 


Every outbuilding is required to have one. 
Your wiring has been done in the style of a separate service, even though it's more of a split service with the house.  Not only does a service need a GES, it also needs a Neutral-Ground equipotential bond to peg neutral to earth potential. Yours has none of that. 


So you need to drive 2 ground rods 8' long, at least 6' apart and preferably on opposite corners of the building.  Then run some, let's say #6 bare copper wire from the ground rods to the panel. Must be continuous, no splicing.   At that point I would obtain an accessory ground bar for your panel and land them on that accessory bar, then also add a #6 strap from the accessory ground bar to the neutral bar.  This assures you have a neutral-ground equipotential bond.  There's no harm in having more than one as long as it's in the main panel, however, having only one allows a neat diagnostic trick. 

That will bring your "main panel" up to Code.  

I would also move all my grounds to the new ground bar (or to a ground bar tied to chassis). This is not a Code requirement for a main panel (it is for a subpanel), but it allows a neat diagnostic trick.  Now that all grounds are on the ground bar, look for any neutral-ground screws (green) or straps that are on the neutral bar.  You want to isolate neutral from chassis/ground  (except for that #6 strap you installed).  Now, you can put a clamp ammeter around that strap.  It better say 0 amps.  If it does not, you have a ground fault.  Chase that by shutting off breakers until it goes  away, then divide the faulty circuit until you find the particular problem.