ok I solved it

for those, who have the same problem
Here is my problem:I'm using Wordpress, which is installed in the root directory.
My SMF-Board is located in a subfolder /forum.
Wordpress offers an internal rewrite option which is harming my SMF rewrite.
I've thought alot about this problem, but I didn't find a proper conclusion.
I have only got 2 ideas:
This idea works :
deleting the smfcode in the root .htaccess and putt a new .htaccess in die /forum folder