[imp] can't get to the login screen

[ianux]

I would to reach imp directly to http://localhost/ instead of
http://localhost/horde/imp
so I did what was told in the FAQ, but when I try, I get an error in
mozilla :
the connection to http:/127.0.0.1:80 has terminated unexpectedly
I can get the test page http://127.0.0.1/test.php, and
http://127.0.0.1/login.php show me the login screen, but when I try to
login, I get the same error.
What's wrong ? Is this an IMP config error or an Apache config one ?