Ok, first things first. This is not a solution, its an important note.
After more than 3 years since release of the game i had built up my killcount up to about 38k kills in a legit way. But i recently cleared the chache of my harddrive in the Xbox system settings.
Besides removing updates (on any game), this reseted the ingame kill counter. So i have to start all over again. Other ingame counters are effected on ths too by the way.
Just wanted you to know.