I deactivated all plugins via phpmyadmin – in the sitemeta-table as well as in the options-table of the single blogs (there are 3 in my installation right now).
So the HTTP-500-Error disappeared. And all of a sudden the P2-theme worked properly as well.
This means: the regular deactivation via backend of the buddypress-plugin didn#t really work. The umlaut-issue with P2-theme seems related to buddypress.