Siehe Beamer-Doku, Abschnitt 10.6 Adding a Bibliography:
Zitat Zitat von beameruserguide
The environment must be placed inside a frame. If the bibliography does not fit on one frame, you should split it (create a new frame and a second thebibliography environment) or use the allowframebreaks option. Even better, you should reconsider whether it is a good idea to present so many references.