Why is it over constrained.

1) depends on the 2 mates and the geometry mated to them.

2) What do you mean by "None of the parts are downloaded".  I do not understand this.

3) Can you share the assembly (pack and go) here?  How can I attach a file to a forum post?

Could it be that both parts are fixed?

A simple guess:

Is one or more of the sketch segments having issue(s) with the fact that the vertical or horizontal constraint is automagically added to lines that have those properties, when you add your own contraint?

It can be how it is mated. If you mate the surface to the edge it can constrain the part in both.

I was having this problem as well. I figured out it was because I was constraining that part to another added part (within the part file) instead of the surfaces of the original part. Example: Constrained a bolt into a hole of the original part. I tried to constrain the nut to the bolt with a concentric mate, but it didn't work. Once I did a concentric mate between the hole in the part and the nut, it worked!