OK, I solved the problem for myself:
(The paths are in german. They may vary in english OS)
Go to Programme / Gemeinsame Dateien / InstallShield
and remove (or better backup before) this folder.
Reinstall.... this worked fine for me.
Is it the fault of obsidian?
No and yes. Primary it is the fault of InstallShield, but obsidian should not use this install-tool if it doesn't work correctly.
Anyway I can play now.
Regards
Tosek