Olas,
I had this same problem with sslstrip last night. I ran the update / install again and it worked today.
It seemed like they change the Packages (http://downloads.openwrt.org/snapshots/trunk/ar71xx/packages/Packages) file before they update the ipk's. I think this is where the disparity between the md5sums came from.
If this happens again for you. You can always download the ipk from "http://downloads.openwrt.org/snapshots/trunk/ar71xx/packages", check the md5sum, edit /var/opkg-lists/snapshots to reflect the calculated md5sum and try installing again. I would reccommed against this, just in case there is a problem with the ipk.
-Luke