First thing I stumbled was the error of loading excalibur. It seems
avalon-framework package has it's jars not anymore below excalibur/ .

As workaround I did

 # ln -s .. /usr/share/java/excalibur

the next error I am unable to solve yet is this one:

 (cd /home/adrian/git/open-build-service-documentation/build/.tmp/ &&
/usr/share/daps/libexec/daps-fop
/home/adrian/git/open-build-service-documentation/build/.tmp/book.obs-admin-fop_color_en.fo
/home/adrian/git/open-build-service-documentation/build/obs-admin-guide/book.obs-admin_color_en.pdf
 )
log4j:WARN No appenders could be found for logger
(org.apache.xmlgraphics.image.loader.spi.ImageImplRegistry).
log4j:WARN Please initialize the log4j system properly.
Exception in thread "main" java.lang.NoSuchMethodError:
java.nio.IntBuffer.flip()Ljava/nio/IntBuffer;
        at
org.apache.fop.fonts.MultiByteFont.mapCharsToGlyphs(MultiByteFont.java:685)
        at
org.apache.fop.fonts.MultiByteFont.charSequenceToGlyphSequence(MultiByteFont.java:571)
        at
org.apache.fop.fonts.MultiByteFont.performSubstitution(MultiByteFont.java:552)
        at org.apache.fop.fonts.LazyFont.performSubstitution(LazyFont.java:424)
        at org.apache.fop.fonts.Font.performSubstitution(Font.java:471)
        at
org.apache.fop.fonts.GlyphMapping.processWordMapping(GlyphMapping.java:141)
        at
org.apache.fop.fonts.GlyphMapping.doGlyphMapping(GlyphMapping.java:92)