"Not Found" error uppon HTTP login attemt

[KrissO_85]

When I try to access my ftp via http://domain: port I get the main page with the
logon and the pass field. I enter the correct details and click "Submit".

It redirects to http://domain: port/login.cgi and then simply shows the following message:

"Not Found
The requested URL was not found on this server."

You all know this error. But why does it pop up here? FTP access works fine and Telnet accessworks fine, but aparantly not HTTP.

Suggestions? Thanks in advance