Der Ami makes the excellent point that the substance could be excess rosin flux. That would also melt and become sticky, but it would happen at a lower temperature than cured epoxy. As he said, rosin flux has a distinctive smell. Just going by the color, that's what it sounds like. Maybe you could narrow it down by trying to dissolve some in turpentine or mineral spirits. Cured epoxy should not dissolve in either.

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