OK, looks like it was installed too tight, had dirt on threads, or cross-threaded (not too likely unless it's leaking). Use the belt wrench method Caco suggests and install a rod or long screwdriver in the buckle for leverage. Don't use too much force or you'll break it. Instead bump it lightly and move around the ring. With a little patience it will start to move then you're good. Another method is to do the same thing with a block of wood and rubber hammer. Work your way around at every other notch. If you have trouble try doing opposite sides one after another. Trick is moderate force and patience. After you get it open inspect/replace gasket then lube very lightly with silicone then hand tighten only.
I hope this helps.
Chris
