When trying to print a test page from the cups frontend on localhost:631 it said Unsupported format ‘application/postscript’! (German: Nicht unterstütztes Format ‘application/postscript’!).

Easy: add those lines to /etc/cups/mime.convs (or uncomment them):

application/octet-stream application/vnd.cups-raw 0 -
application/vnd.cups-postscript application/vnd.cups-raster 100 pstoraster