Done.

All speculations about this issue revolve around inability to read existing config file at some point for an unknown reason.
Did you test this change in the given situation?

When I removed all permissions from config file, FileExists() returns false because _wstat64() returns -1.
So, the added code doesn't handle the advertised error condition, at least on Windows.

No but _stat is valid in order to check for file existence in theory. In case it returns -1 it's becouse the file doesn't exist or a parameter is not valid. I wouldn't be surprised if it's another bug in _stat. I could try to force the internal version in LZDoom, i'm using VS 2017 BTW. On the other side ::exits internally uses GetFileAttributesW.

This is not what I was asking about. I see no point in change that doesn't fix the problem even if assumptions are correct. The latter wasn't proved, but this doesn't matter if the added code are not called anyway.
To put it clear, a theoretical fix for a theoretical problem is not how things work.

The problem is real, what happens when you remove the permissions from the file? I assume the config is still overriden, i will do some testing in that situation then to see if it's a bug in _stat or ::exists work. Well we would need to change the permissions again before exiting of course.

Well, i've just confirmed that _stat is broken on windows and the internal version works. I see it's still in GZDoom guarded by #ifdef USING_V110_SDK71 and it's _wstat64i32 but now _wstat64 is used instead.