INIT_SP_FROM_STARTUP_DESC is not a function, it's a macro. It expands to some inline assembly instruction loading the initial stack pointer.
It's not mentioned in your text, so I wonder if it is clear that _Startup is not an ordinary function, it is the entry point of the code usually pointed by the reset vector any maybe other vectors too (COP,...).
So I'm also not clear what exactly you mean with:
>I have stepped through my code and the error occurs at a function called
The point is, _Startup is executed before your code, so if you reach it after your code did execute, something bad did happen, maybe a COP reset? Maybe something else, but you should not get to _Startup after stepping through your code.
Daniel
PS: In Processor Expert setups, some PE generated initialization code gets executed before _Startup, but user code still happens afterwards (in usual setups).