I forgot to set errcheck:

    import ctypes

    userenv = ctypes.WinDLL('userenv', use_last_error=True)

    def errcheck_bool(result, func, args):
        if not result:
            raise ctypes.WinError(ctypes.get_last_error())
        return args

    userenv.CreateEnvironmentBlock.argtypes = (ctypes.POINTER(ctypes.c_void_p),
                                               ctypes.c_void_p,
                                               ctypes.c_int)
    userenv.DestroyEnvironmentBlock.argtypes = (ctypes.c_void_p,)

    userenv.CreateEnvironmentBlock.errcheck = errcheck_bool
    userenv.DestroyEnvironmentBlock.errcheck = errcheck_bool

    # ...