sorry for my late answer...
in sql 2012 the reporting services didn´t work.
i reinstalled the database on a sql 2008 R2 / default instance. after that, i configured the reporting services (programs -> sql 2008 R2 / configuration tools / reporting services configuration manager)
-> that worked!
one thing that i read in a other thread was, that there are problems with reporting services of the operations manager and configuration manager on the same server.