ok... nokia hands showed up again... i deleted the default.conf and made a new one in x-term with vim.... and taaaadaaaaaa... there it was
(maybe there was something wrong with the copied default.conf?!)

after that i edited the file and changed the name behind filename= to maemo.avi but that doesn't seem to work at all. :/

EDIT:
after all it worked but i don't know why it didn't at the first attemp. ^^
i only copied the video again to MyDocs and moved it from there to /opt/media/ (which i symlinked to /usr/share/hildon-welcome/media/) and blapp.... the video showed up smoothly

strange behaviour though.... but now i'm happy ;D