After a few hours I have found an article which descripes a
similar problem with the installation of Domino 8.0 beta ...
I don't know why, but the file stcommsrvrtk.jar was absent :-(
[This Java-package also contains the missed STObject-class]
As solution, I copied this file from a Notes-/ Administrator-/ Designer-installation to the Domino-folder (*\Domino\jvm\lib\ext) !!!
After a re-start of the server, DWA with IM and retrieving of the "Online-List" is able ...
Greeting from Austria,