The thing is, if it's on, it's bound to screw something, sometime.
Maybe the switch from Winderz to Apache was where this cropped up?
Tell ya what... What if you ask them to disable it, temporarily, just to prove whether or not this is what the problem is?
If they disable it and everything works, we'll know what the problem is.
If they disable it and the problem persists, we'll know that we have to look at something else.
But, as you haven't changed anything and this hassle seems to have appeared at around the time that they switched systems, it seems, to me, to be pointing to that being the problem.
In all honesty, I can't think of anything else that could be causing this, at all.
That doesn't mean that it might not be something else. Someone with a bit more knowledge than I have might have other ideas. But, for me, this is really screaming "mod_security"!!!