I see LuaSocket pulling the right zsync control file, and that's the only device-dependent part, so, that, at least, works (which we knew already, since you do see the 'new version available' prompt

).
tar might be falling on its ass on Kobos, that's the next step.
Easy enough to check with a shell: as the same user than KOReader uses (root? not familiar with Kobos), and from the koreader directory (again, not familiar with Kobos ;p), run:
Code:
./tar cvf ota/koreader.installed.tar -C .. -T ota/package.index --no-recursion
echo $?
And report back

.