Kiessling originally said that he did not see the goal, but has since taken to Facebook to apologise.
"I can fully understand the reactions from many of you and I myself am absolutely gutted," Kiessling wrote on his official Facebook page.
"After seeing the replays on television, I can clearly see that it was not a legitimate goal.
"I'm sorry for all sports fans. Naturally, it's not nice to win like that. Fairness is important in sport, for us in the club and for me personally."
The German Football Federation have said that they will consult FIFA over how to handle the situation.