I just committed my fixes. Apparently, there are several postgres upgrade scripts for acs-content-repository after version 5.1.2, but those were never transfered to oracle (grrrr!!!). So I had to add "security_inherit_p" to content_folder.new, content_item.new and acs_object.new (get that!!! nobody thought about upgrading the acs_object-package either!!!). In addition to that, content_folder.new needed the parameter package_id for it to work with the new content-repository code.
Still, after those changes I got the error mentioned by Malte which points to a problem deeply rooted in the content-repository: how to deal with lob-values on insert/update of a revision through the nice tcl-interface. I could work around the problem for now by changing the appropriate procs in assessment not to provide a value for lob-columns in a call to content::revision::new and to update them later in the normal way to do for lobs, but I think this will not fix the deeper problem in the content-repository and since this is the foundation on which openacs is build nowadays, I think it needs fixing asap.