[scribus] German Umlaute - red squares

[maltefiala]

Using ubuntustudio 8.04.1, kernel 2.6.24.19-rt , some fonts (purisa, and 
others) in scribus show umlaute as red squares, altough they show up as normal 
in Specimen-Font-Previewer.

Do you know how to make it work?

all the best,
malte