[Featureserver] problems with OpenLayers WFS

[G. Allegri]

Hello list.
I'm trying to setup a FS WMS service to be called by OL. I setup FS
with a "yyy" layer conected to PostGIS. It works correctly with a base
request like:

> http://85.18.124.255/iet/IETmapfish/client/fs/fs.py/yyy/?format=WFS
or
> http://host_url/iet/IETmapfish/client/fs/fs.py/yyy?format=WFS

(both of them work...)

But if I do:

> http://host_url/iet/IETmapfish/client/fs/fs.py/yyy?format=WFS&maxfeatures=600&SERVICE=WFS&VERSION=1.0.0&REQUEST=GetFeature&SRS=EPSG%3A3003&BBOX=1437940.723214286,5056734.944196429,1903269.276785714,5159996.055803571

I get an "unhandled request return internal server error" inside OL,
and if I call it outside I get:

An error occurred: current transaction is aborted, commands ignored
until end of transaction block
  File "/www/projects/iet/lib/featureserver-1.11/FeatureServer/Server.py",
line 186, in modPythonHandler
    request_method
  File "/www/projects/iet/lib/featureserver-1.11/FeatureServer/Server.py",
line 145, in dispatchRequest
    result = method(action)
  File "/www/projects/iet/lib/featureserver-1.11/FeatureServer/DataSource/PostGIS.py",
line 204, in select
    cursor.execute(str(sql), attrs)

What's wrong?
Giovanni