i checked the permalinks when buddypress is disabled.
default works, the rest doesn’t.
i have, however, other wordpress-blogs with the same hosting-provider and they do have the pretty permalinks.
trying to copy the htaccess file from a working blog, didn’t help.
i guess this is not really a buddypress problem after all?
update:
i added index.php to the permalink ( /index.php/%year%/%monthnum%/%day%/%postname%/) and now the blog works. with the permalinks. but still no buddypress.