I have about the same problem. "Preview using Lilypond" yields at first
a Kdialog window stating
ERROR: LilyPond processing failed.
LilyPond output follows:
... and no Lilypond out follows. "OK" on this window then yields a
Kdialog window stating "Processing LilyPond file..." and nothing more
happens. The same happens when I use "Print via Lilypond".
Exporting to Lilypond gives a Lilypond file which usually can be
compiled without problems. But not always. For instance I had such a
file where Rosegarden only had marked slur endings, i.e. ")", but the
beginnings "(" were missing.