http://bugs.winehq.org/show_bug.cgi?id=25529
--- Comment #2 from A Wine user RandomAccountName@mail.com 2010-12-16 10:05:56 CST --- (In reply to comment #1)
You have to make sure the 64-bit version is set correctly ("wine64 winecfg").
I see now. Running it with ~/wine-git-x64/wine64/loader/wine64 gives "internal error" while running with ~/wine-git-x64/wine64/wine gives the unsupported OS message. Is that how it's supposed to work?