This seems to work for me, so lets try and do it manually and see where it goes wrong...

Take a sample document that doesn't work and first try...

a)
mkdir /tmp/foo
cd /tmp/foo
unzip /path/to/sample.odt
gthumb Thumbnails/thumbnail.png

and see if there is a thumbnail, and if it is viewable. If that works then...

b)
gsf-office-thumbnailer -i /path/to/sample.odt -o foo.png -s 300
gthumb foo.png

and see if that works

I unzipped the document and was able to view the thumbnail (was not able to read it even when zoomed in). I ran gsf-office-thumbnailer and this is what I got

$ gsf-office-thumbnailer -i Network\ Survey\ Checklist.odt -o foo.png -s 300
calling convert 'foo.png.C9MPFU' +matte -thumbnail 300x300 png:'foo.png'
error: Failed to execute child process "convert" (No such file or directory)

It created the foo.png.C9MPFU file that was viewable but unreadable.

Aha, so what I think we have here is a missing dependency on ImageMagick on that /usr/bin/gsf-office-thumbnailer

if you install ImageMagick does it "work" again ?

Yep, that fixed it.  I didn't think to see if anything provided "convert".  I appreciate you getting to this so quick.  It really wasn't that important.  Just something minor that wasn't working.

Fixed in rawhide by moving the convert tool to the gnome subpackage and adding a dependency on ImageMagick.