Regarding your flexsdk.target, do you actually have a 10.x.x in [flex-install-path]/frameworks/libs/player? My flex install only has 11.1 so I've flexsdk.target=11.1. Just wondering if you've the latest install. This is on a Linux box so no "execextension = .exe". I've not tried building jwplayer on a Windows box though.
Thank you for this idea.
I have both a 10.1 and 10.0 folders in my [flex-install-path]/frameworks/libs/player directory.
I have tryed changing the flexsdk.target parameter to 10.0 or 10.1, but the result is the same : build fails with the same error message.