As I tried to emerge mldonkey-2.8.2, it failed. Unfortunately, I don't have the error-message anymore, was something like: I can't compile this and that. So what I did was upgrading ocaml from version 3.09.2 to 3.09.3, which is unstable at the moment and has to be written to /etc/portage/package.keywords - and everything worked well after that. So I guess, mldonkey depends on that version. I also ran the ocaml-rebuild.sh script.
*** This bug has been marked as a duplicate of 159461 ***
Dupe.