"Update paused" though complete; game will not run

[Neon Kitten]

After the near-17GB patch yesterday, I went to run The Witcher 2 EE just now and got an error message: "An error occurred while updating The Witcher 2: Enhanced Edition (Unspecified Error)".

I notice that the game update's download, which was some 98% complete last time I saw it, now says "Updated: 0 bytes /0 bytes". The download cannot be resumed. Interestingly, even while downloading I noticed that the data was not being put in the "Steamapps/common/the witcher 2" folder, but somewhere else. Instead of "updating" the text above the pause/resume button said "Pre-loading".

I tried verifying the integrity of the game cache. Nothing happens. The dialogue box just sits there "verifying 1 of 1" with zero disc activity.

I tried changing to the latest beta version of Steam. Same problem.

I tried changing download servers. Same problem.

I tried running the Steam exe as Administrator. Same problem.

I'm all out of options. Any suggestions?

(Please note: "download it from GOG" is not a helpful suggestion!).