I saw some post here on forum, that someone had this problem too, so I am glad that i am not the only one.
I think you are referring to this case:
https://bitcointalk.org/index.php?topic=5291882In that case, the problem was due to a bug and they fixed it.
The problem here is that you didn't download the letter of guarantee which is the only thing that can prove the address to which you sent the 15
BTC belongs to mycryptomixer.