I may be stating the obvious here, but if you have enough thread to grab with pliers, don't use pliers, use two nuts and use a wrench on the inner nut against the outer nut. If it was so tight that it broke, pliers won't do any good. Again, maybe that is all you guys do, but I could not help since I see pliers being mentioned more than once.

 

Must be the weak point. I have the same leak (same spot), though I have just been globbing some hylomar blue in the area every few weeks (after recleaning it) until the coolant starts to dribble. It's a really SLOW leak, but will inevitably need to be fixed correctly. I'll probably do the job in the next month or so, before it starts getting warmer out.

I think you are going to need to pull the cover as suggested and replace the seal/gasket if you want it done right.