Well, its funny, but I couldn't find a solution for this problem.
I use several plugins, which alteres the content in some way, i.e. wpDefine, SH-Autolink, IMGPop and so on.
wpDefine for example produces alt text and special css code around given keywords, SH-Autolink replaces several keywords with a given link. Well, both together with the same keyword, won't work.
Anyone got a solution for this add_filter problem or is it a general WP pluginproblem that has no workaround/fix?