In T315737#8187139, @Umherirrender wrote:

The failure has happen when building the search result. There must be a invalid title on the wiki which is part of the search result for that word.

Try to run cleanupTitles.php (first with --dry-run) to find that bad title. Or namespaceDupes.php if the namespace config was changed in the past.

It could be possible there is a page_id in revision, which is no longer part of the wiki (broken delete etc.), not sure if deleteOrphanedRevisions.php helps for that (run with --report first)