Yours is the first time i've ever seen that message happen on a Zen Cart site.

And it's not caused by Zen Cart itself. It seems to be something your server is doing at the PHP level, before Zen Cart can do anything.

You said you're not aware of any "changes". But, since the symptoms ARE a change, that means something DID change. Now you need to find out "what".

I'm going to recommend that you do a complete audit of all the files on your server, comparing them against a clean backup ...... you do make backups, don't you? If you never made a backup of the files on your server, then use the original Zen Cart files as the comparison point.
Full process explained here: http://www.zen-cart.com/wiki/index.p...ing_From_Hacks ... go through that until you're certain that you've confirmed that these symptoms are not caused by something malicious.