Possibly the version of MSYS and its libraries changed when the windows nightlys went to GTK3.
Umm, that brings up another point, AFAIK there have been no releases for Windows with GTK3, 1.37.1 is GTK2 AFAICT, but the nightlys are GTK3, so they can't simply be untared on that release. So what are you "installing" them on?