If this happened to me I'd just remove the One-Click panel and try a reinstall.
Rename domain.tld -> domain.tld.old
Make new directory domain.tld
Remove One-Click MediaWiki from list
Install MediaWiki to domain.tld
Should you prefer to troubleshoot:
First, check your Support History to (re)read any notification issued by the installer script after it finished installing MediaWiki, then check (via SFTP) the files in whichever domain you installed to, paying attention to the "owner" field - which should be the same as your SFTP login user name.