You haven't made it clear what your page suffix is - I am guessing it is a simple .shtml file, which it should be. i.e. the file on which the tag is, the file giving the processing error.
I think that if it was working fine and then suddenly stopped there is only one place to look - the configuration. In httpd.conf is where the server is instructed to treat .shtml (or whatever suffixes you/admin choose) as an executable.
If it suddenly stopped working after years of working fine and there were definitely no changes at your end it makes sense to go look at the configuration file. The only way I can think of that it would start serving that error message would be if the .shtml suffix has, for some reason, ceased being treated as an executable.
The support of your host are a bunch of talentless gits. You should push them harder, talk to the sales team and tell them that if this isn't resolved immediately you'll move to a better host. They are definitely to blame. Crazy excuses about caches are just some random rubbish. I remember some gits tried to rob me by taking 8 dollars from me to sell me 1000s of hits, but the hits never came and they sent me a stupid email saying that their delivery system was so fast that ordinary tracking systems would miss it!!!!! I swore at them quite a lot. Then I told paypal to give me my money back - after the briefest of investigations, paypal gave me my money back.
If it was working fine and then suddenly stopped, I reckon it must be the httpd.conf file. If that doesn't have .shtml set up properly, I don't know what does.
There is one thing that I find weird with your stuff - .shtml pages contain a tag like yours, but that tag should lead to a cgi script, not another .shtml. So what is the coding on your other .shtml files which is being called up? I think personally that's what I find very odd about your case. Maybe it is partly at your end - maybe you were getting away with bad programming and an update to their server has closed that hole. Maybe all you need is for your include tags to be pointing at backend scripts, since that is what you are supposed to do...
http://httpd.apache.org/docs/1.3/howto/ssi.html
Quote:
Basic SSI directives
SSI directives have the following syntax:
<!--#element attribute=value attribute=value ... -->
It is formatted like an HTML comment, so if you don't have SSI correctly enabled, the browser will ignore it, but it will still be visible in the HTML source. If you have SSI correctly configured, the directive will be replaced with its results.
The element can be one of a number of things, and we'll talk some more about most of these in the next installment of this series. For now, here are some examples of what you can do with SSI
Today's date
<!--#echo var="DATE_LOCAL" -->
The echo element just spits out the value of a variable. There are a number of standard variables, which include the whole set of environment variables that are available to CGI programs. Also, you can define your own variables with the set element.
If you don't like the format in which the date gets printed, you can use the config element, with a timefmt attribute, to modify that formatting.
<!--#config timefmt="%A %B %d, %Y" -->
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Today is <!--#echo var="DATE_LOCAL" -->
Modification date of the file
This document last modified <!--#flastmod file="index.html" -->
This element is also subject to timefmt format configurations.
Including the results of a CGI program
This is one of the more common uses of SSI - to output the results of a CGI program, such as everybody's favorite, a ``hit counter.''
<!--#include virtual="/cgi-bin/counter.pl" -->