It’s a WP system, but there is a non-WP forum installed in a subdirectory.
I am guessing that the WPFC entries in public_html/.htaccess are messing up how Apache handles what is in public_html/forum/.
Sorry, I know we have a weird setup. The forum should never have been installed in that subdirectory, but 10 years ago the people who did it thought it would be okay.
This is the first time we’ve tried to add a cache plugin to the WP site, so we’ve never seen this problem before.
I thought Exclude Pages would fix it, but I guess it’s more complicated than that. I’ll try a few experiments and maybe I’ll find a clue.