i resolved it more or less myself. we did a complete deinstallation and installation of a standalone install. now almost everythings works (app pool running with local system account though).

We can start the workspace but when we look at a process overview the following error appears:

An error has occurred during report processing.

• Cannot create a connection to data source 'BLACKPEARL'.
  • No connection could be made because the target machine actively refused it

We openen SQL reporting services and looked at the datasources. well, the k2 installation has added a datasource BLACKPEARL there, but in the connection string there was no server name. There was: "Server=domainname.com;Port=5555;autoalias=False" (connection type SOURCECODE)

Does someone have a solution?