This error is not related to the postgres version but to some problem in the internationalisation code (I think). What version exactly did you try to install and how? By 'taking apart 4.5' do you mean you deleted all the files and the database and installed from scratch or did you somehow install over the old version?
Maybe it is a bug in the development version of OpenACS that you got from CVS. If you want to install a stable version of dotlrn than go for the oacs-4-6/dotlrn-1-0 branches, as described in a comment at the bottom of https://openacs.org/projects/dotlrn/dotlrn-doc/dotlrn-installation.
Anyway, the latest 7.2.x postgresql version is recommended. 7.3 still needs some patches to work with OpenACS as you can see on other threads on this bboard.
