[Plugin: WP Geo] markers don′t show up

Hi,
I just installed the wpgeo-Plugin and everything worked out fine – almost.
The maps are displayed in sidebar (widget) and also in the posts, but in both cases without markers. The marker is there on the editing-site but not visible on the post/widget. It seems to be there but unvisible, because the tolltip shows when hover over.
Thanks to anybody, who has a hint for me.

Michael