Dear Gustaf, thank you for your reply.
The version I used is OpenACS Core 5.2.3, published on the following URL: http://www.openacs.org/projects/openacs/download/
I selected it because it was the most recent (non beta) version available.
Anyhow I did try to get the latest version from CVS HEAD and everything worked fine. So the combo I have now is:
Postgres-8.2.0-1 together with OpenACS-5.3.0b1.
Thanks for your tips.
Maurizio