Lilypond print preview doesn't work in Ubuntu 9.04

When I press Preview with Lilypond under the File menu, I get :
  KDialog
  ERROR: LilyPond processing failed.
  LilyPond output follows:

The output is totally blank giving no indication what the problem is.
On the other hand, I can do Export -> Export lilypond file ....
and it will export the file. Then I can run lilypond in a terminal on the exported file and that will give me pdf and ps files from my input ly file. It is only the Preview with Lilypond which fails.

The other problem is sometimes the preview command works. I haven't found any connection to find out what causes it to work or fail.

I was wondering if I could somehow put a debugger on this to find out what is going on. Likewise I would like to know if this is a known bug?

Ilan