Thread:Project:Support desk/Hooks array for event 'ArticleDeleteComplete' is not an array!/reply

As said on those forums, it's much likely an Extension which you downloaded for a different MediaWiki version that the one you're running.

Try disabling all extensions and try deleting the page again. If the deletion success, then enable all extensions again and delete a test article, disabling one extension each time until you find the offending extension, then download it again for 1.18 release.