Thank you, Tomas, for your answer. Unfortunately it doesn't help.The installationpath is opt/joomla-3.3..6-0, but there I don't see installationfiles to be deleted.
In the Joomla's administrationpanel I see this message:
Congratulations! Joomla! is now installed.
PLEASE REMEMBER TO COMPLETELY REMOVE THE INSTALLATION FOLDER.
You will not be able to proceed beyond this point untill the installation directory has been removed. This is a security feature of Joomla!
Underneath a button: Remove installation folder. But when I click this button I got the message:
Error. Installation folder could not be deleted. Please manually delete the folder.
Down under in the screen two buttons: Site and Administrator
Clicking on the one or the other gives only the same screen.
If you can help me, I'll be very grateful to you.