I just upgraded my MacMini Server from Lion Server to Mountain Lion using OS X Server. I am having the same problem with PostgreSQL that I did last year when I first instal
I was able to solve by simply filling in 127.0.0.1 for the PostgreSQL host address rather than leaving it blank. (Django Example)
DATABASES = {
'default': {
'ENGINE': 'django.db.backends.postgresql_psycopg2',
'NAME': 'database_name',
'USER': 'database_user',
'PASSWORD': 'pass',
'HOST': '127.0.0.1',
'PORT': '',
}
}