Actually, I've now manually installed all other pkgs including the latest CSSU thumb upgrade. So it's just the libx11... package, which perhaps makes my comments off-topic ? but is there any reason both of my n900s are unable to update libx11-data_1.1.99.6+0m5+withxcb_all.deb ?
I don't know what this package is, but it is not part of cssu-thumb. Maybe try to remove it, to see what application has pulled it as a dependency.
dpkg --force-all -i *.deb