Markus,
Yes, unfortunately, that's a bug with our plugin right now. The problem is that K2 will generate the meta description tag for your site automatically, and if the Like (or any) {JFBCxxx} tag is part of the generated meta data, then Facebook inserts its widget there. The best solution, for now, is to manually specify the meta description in K2 for your articles so that the {JFBCxxx} tag isn't sucked in automatically.
Hope this helps, and we do plan to fix this in an upcoming release so we can promote better K2 integration. Good luck!