Hm, sounds strange. I've never tried it out on an installation without MySQL only with both MySQL and PostgreSQL. My guess is that the mysql driver is activated somehow before the postgresql driver is used. Mysql is the default choice in the site.ini file.
Could you try to edit site.ini and change the
DatabaseImplementation=ezmysql
to DatabaseImplementation=ezpostgresql
We'll have to try this out a bit to figure out where the problem lies.