Upon watching replays of the dissalowed goal and using heidelbergs own corner footage. The goal scorer is an offside position when the corner is taken, of course you cant be offside if you score directly from a corner, however another heidelberg player heads it to the player in question who returns back to an onside position and scores. I have no idea what the rule is but has the goal scorer been flagged for coming back from his initial offside position?
Edit: Again using bergers footage, if you pause it right on the header the scorer is offside.