This is a common bug, for those who have been playing v1.07 at least. It has been SOMEWHAT fixed with v1.084. There is a current thread about this in the CDProjekt forums, for some reason I can neither log on with my GOG nor register a newe one. Would appreciate if someone would quote the following in that thread for me.

Here is what I rapported back to support:

Well, it fixes it partially. You can talk to the Sergeant, but the stable boy is missing. Did some backtracking and concluded this: The quest The Last Wish causes the bug that makes the stable boy vanish. Solution: Talk to the Sergeant at once after the Ciri questline permits it, make sure the stable boy is activated, but do The Last Wish before actually talking to him, as talking to him will negate that quest. Forum registration is not working, so I cannot communicate this to the others in the thread that is active about this; you might want to.

Here we go, after countless hours i managed to get the Stablemaster spawn for real.
If you're stuck on mainquest: Ugly Baby and the Sergeant only gives: Nothing here for you...
There's only one way to do it and it is to backtrack to the quest: Nameless.

1. started with a fresh install (gog-preload) version 2.0.0.28
2. then patched straight to latest version 1.8.4
3. loaded my save to redo the quest : Nameless

Immediately after completing this quest (Nameless), the quest Ugly Baby will begin.
You can then teleport to the Crow's Perch and talk to the Baron's Sergeant.
After, you go see the Stablemaster and the bug as mysteriously gone!

P.S.: i've tried all kind of saves, none will do unless you redo the quest: Nameless.

That's it that's all.
Have fun.