I'm no expert, but it looks to me as though that branch is dead.
It's difficult to know what's affected it, but if it were my tree I would cut the branch off, making sure that I cut right back into healthy wood.
I would then burn the branch, just in case it has an infestation or infection of some kind.
Most advice nowadays seems to suggest that it's best *not* to paint over the cut with a sealing compound. Just leave it open and allow it to heal over naturally.