Over the weekend I've received a number of messages (e-mail, comments, what have you) about a sudden re-appearance of the game's "this is a demo" screen in v1.8.2. The problem is that this demo screen is what you get instead of the next level in the game. Oops!
As best I can tell the problem has something to do with a feature I added (support for running Shifter on Demo versions of the game) and the game attempting to access the maps directory, which is how my addition works. For some reason, some people's installations of Deus Ex CANNOT access its own map directory in the way my addition needs to in order to work. This results in it thinking the map it's looking for doesn't exist, which makes it think its the demo.
Okay, highly technical explanation over. Bottom line: I am fixing it. v1.8.3 will be coming out in the next few days with that fixed, plus some other bugs which have been brought to my attention. In the meantime I can only suggest two things: downgrade to v1.8.1, or try running the game as an administrator.
Oh, you can also check out this link for a beta which SHOULD have it fixed.