Qt creator: Error trying to open designer

The problem appeared when I made a fresh install of debian lenny and then downloaded and installed the qt sdk from the testing repositories.
When I'm trying to open a *.ui form from inside the creator choosing the qt designer as an option,
I get this dialog error message:
"Unable to create server socket: The address is not available"

Note that I can open it using the form editor, plain text editor or binary editor. Designer works perfect if it's called from the menu.

Any idea what maybe wrong??