Let me start by saying this is NOT an installation problem, inasmuch as this is a standing website that's been there for years. The error is not happening when accessing any of the normal parts of the website, but rather when accessing plugin files. Let me also say I'm perfectly aware that this must be a server config problem, since I've got the same plugin running on two separate installations of WPMU on the same VPS: same version of both, and it's working on one but not the other.
The plugin in question is cforms. What's happening is that when the plugin accesses any of its stand-alone files such as lib_ajax.php, it returns the "No WPMU site defined" error instead of the file. If I browse directly to the file, I get the same. Every file in that subdirectory is the same, yet the next folder up is not.
What I need from the group, if someone will be good enough to indulge me, are some leads as to why the error occurs in a general sense. I'm not seeing too much looking at wpmu-settings.php that leads me anywhere.