@munsell Quick -- maybe dumb -- question: was the wiki you were attempting to install 'Semantic Mediawiki'? That isn't a full wiki installation; it's an extension to install over top of an existing normal wiki installation. (It's confusing because initially it sounds like a drop-in replacement if it's the first time you're doing it... and I know it tells you to install it through composer, so that's why I'm asking.)
I'm not entirely sure how to fix it, but if that's what happened, one of the more server-savvy folks around may be able to help you resolve it.