Turns out, as of a few versions ago, #PmWiki has silently introduced a dependency on the intl extension... which is no longer included in #PHP 8.2 and later, and not installed by default on cPanel hosts. For now I can still switch back to 8.1 on the impacted subdomain. In the long run, I'm screwed.
@notimetoplay This seems strange on all fronts. Is there some replacement that 8.2 is deprecating intl? Wait, not included? Is it obsolete? Also strange to add a dependency without making it clear. Good luck.
@JigmeDatse Thanks. No, it's just an extension that used to be installed by default so everyone's come to depend on it, and apparently never heard it's not there anymore.