After enabling the Wikibase Client extension, as instructed in T5510, I get a MediaWiki internal error:
MediaWiki internal error. Original exception: [a560b73641f49a7ef4cf27a8] 2020-06-03 10:32:26: Fatal exception of type "Wikimedia\Assert\ParameterAssertionException" Exception caught inside exception handler. Set $wgShowExceptionDetails = true; at the bottom of LocalSettings.php to show detailed debugging information.
The wiki is https://nonbinary.wiki/wiki/Main_Page