Probably the easiest thing to start with it to clean that spot of the chain really well, using whatever you normally use.....(Kerosene, WD40,etc)
Then start working that section of the chain sideways, and up/down, maybe using more cleaner. I'd use some safe solvent type stuff....WD40......with the hopes of washing out whatever may be binding it up.
(Grab with your hand....you're attempting to dislodge or grind up anything in the links)
You may be able to work loosen some dirt or sand, or rust, if that's the problem.
If you can't get that section of chain free, and running well........I'm out of suggestions.
Usually this works with even worn chains, for a while.....