They say:
Über Jahre hinweg ist die Bitcoin-Börse Mtgox über die sogenannte Transaction Malleability bestohlen worden. Insgesamt seien es 744.408 Bitcoins
It means, that there was a bug over the last years and Mtgox does not realize it.
No, they don't say mtgox didn't realize it. On the contrary:
"Dokumente sollen belegen, dass Mtgox darüber Bescheid wusste und ein Ausstiegsszenario plante."
says, they knew about the bug and were planning an exit scenario.
...aaaand they know this for sure from...? And I guess they have proof as well? No? Hm.