If you go to the Tools tab in the AdminCP, is there a pending cache rebuild that tries to finish? If not:
It sounds like the phrase-cache rebuild and the admin-template-cache rebuild that were triggered by the installer have not been applied to a higher level cache that you have. If you use a XenForo add-on that caches phrases and templates (such as [bd] Cache or DragonByte Optimise), try flushing that cache.
I have also seen a similar problem in one version of PHP 7. It was possible to reset the cache by restarting PHP. The issue also stopped occurring for upgrades after upgrading PHP to the next version. I do not remember what exact PHP version it was.
Most recently, a user told me this was related to CloudFlare, and that flushing Cloudflare's cache resolved the issue.
This issue is sometimes limited to the IP that installed the wiki. When some customers who reported this issue submitted a private ticket, the admin panel loaded correctly for my IP, and was magically fixed for their IP the next time they visited.