Apparently, as a result of my toggling PHP up and down versions, I have now managed to make it so that it doesn’t matter whether the Asgaros Forums plugin is enabled or not — the “Events” link for The Events Calendar takes me to the same “not found” page.
Tried that just now (I hadn’t thought of that). Unfortunately, I got the same results.
It occurs to me that it may be a PHP issue. I was at PHP 7.4. I changed it to 7.3, then to 7.2, just to see if an older version might be more compatible. Nothing changed.