Emmanuelle: A tip I can give you off the top of my head is to check your oracle db setup statements. You may be missing "javasyspriv" or
something else. Try to install Openacs without the dotlrn packages. My guess is that you will get the same error without the dotlrn packages.
Requisite ad for Lars's tool to help write oracle statements:
http://pinds.com/acs-tips/oracle-statements
If you get the same error without the dotlrn pacakges, post here for help on installing openacs. In the meantime, you can register at http://helice.mit.edu:8080 to see dotlrn from a user's point-of-view.