I’m not sure is it corresponding with SDL_systimer.c function Sleep(). SDL was built on my dev pc with VS2013 Update4.
Windows has it’s own Sleep() function/API, that until recently (as of VS2013 Update 4, in fact), was not available for use in Win 8.1 apps (it’s now available for use).
Can you make sure you’re using the latest version of WACK [2], and if not, give that a shot? I have version 3.4 of the WACK suite, and was just able to validate a test app, one that did call Sleep() (via SDL_Delay()), and did not get any errors.