Bitter Taste: Beef Stew
It's the tomato paste. Many brands of tomato paste can have a bitter, almost metallic flavor if it isn't fried off first. I don't use it in crock pot recipes for that very reason unless I saute it in some oil for a minute before adding it. You can add some sugar but that doesn't counteract the bitterness. 

Also you are adding too much of it, 2 tbsp for 1 cup of wine is very, very tomato pastey for lack of a better term. You could try half that and see how you get. 

So try a bit less paste, fried off first or use a good thick canned tomato sauce instead.