Another possible fix:
stop nepomuk
Remove the file ~/.kde/share/apps/nepomuk/repository/main/data/virtuosobackend/soprano-virtuoso.trx
restart nepomuk
It worked for me without data loss
Another possible fix:
stop nepomuk
Remove the file share/apps/ nepomuk/ repository/ main/data/ virtuosobackend /soprano- virtuoso. trx
~/.kde/
restart nepomuk
It worked for me without data loss