A few questions first:
1. Have you tried restoring all settings to default prior to rendering and then specifying uncompressed frames?
2. Have you tried playing the file in another media app like Power DVD?
3. Does the file import into a video editor?
4. What’s the file size of the final render? Chances are that if it’s mondo-big WMP is wheezing under the strain or your system is.
5. What’s your system config?
Technically you’re right. There’s no reason why WMP 11 shouldn’t play the file. If I’ve ever come across an error like this it’s usually been an error in the render process. Re-rendering the file normally does the trick. Just select a lossless render at best settings. Manually select uncompressed frames and render the file. Hope this helps. Scream if it doesn’t.