The error message only shows up if the server tries and fails to execute the Include. If the page source still shows the
line, it means the server isn't trying. Uhm. That sounds as if I'm faulting the server. It's just a dumb machine that does what it's told. ;)
This line will be in the page's source code-- View Source or similar in your browser-- NOT in the visible page. (Notice how the format uses "comment" tags.) The default Include error message is visible text; that's one reason you want to change it.
This question has now been bumped over to the Apache forum, so it should pick up an answer from ::cough-cough:: someone more helpful. There's a particular thread that lists all the possible permutations that some server somewhere might conceivably demand before it is willing to do SSIs, but I can't find it. I did come across this one
AddHandler server-parsed .html
which you could always throw into the mix. I've never needed it myself, but it was in a thread from 2010 so it probably won't make your server explode.
Incidentally, you keep saying "blank page". Never mind what you see in the browser. What's important is what you see in the source code (as displayed by your browser). There's a big difference between source code that's truly empty, and source code that contains a bunch of js or php, none of it resulting in visible content. Always look at the source code, because that's what the server sends out.
Philosophical observation: SSIs are extremely useful if you're coming from a background of raw, hand-rolled HTML as I did. They help you get used to the idea that what you see in your code will be different from what the end user sees if they View Source. This is a big conceptual step. If you're coming from a CMS background, you're going in the opposite direction.