I also had the problem with the Polish demo that it wouldn't install and gave an error around bink<something>.<something>. Redownloading, freeing up space on C: and installing in the default location, not sure which one of those steps, fixed it. Didn't need to upgrade DirectX, so it's probably not it.
Been playing around with the Polish demo and seems like they included a smaller piece of the gameworld that I anticipated: only Grenwort(sp?), instead of all of Darion, which would've been more logical.
Perhaps there were filesize considerations, or the fact that the demo would've taken a good 10-15 hours
, but including just one area means that means some of the quests in the demo can't be completed.