Thank you for your answers, unfortunately I can add a topic, but have not permissions for replying to a a topic. So here for short:

Error in Backend Layout Manager
Error in Frontend Menu > Dashboard
This works now fine after installing administrator/com_cjforum/sql/updates/ directly in my SQL database
but caution, after installing cjforum again, it does not work anymore till manually updating sql database. It does not seem to correctly install these sql scripts.

Error on accessing plugins in backend
Error on accessing options of cjForum in Backend
I did not install it on existing CjForum installation.
After all there does not to exist any file joomlaroot/components/com_cjforum/helpers/constants.php in installer oder after installation at this place.