I just updated my mediawiki from 1.30 to 1.34 (including database upgrade and PHP version upgrade) specially for this extension. I did check if the dependencies mentioned on the extension's page are enabled and installed (one was commented out). Checked Preferences - no option to turn it on/off. I've added lines proposed on the extension's page to LocalSettings.php (not sure if in the correct place as I know nothing about php):
$wgPopupsHideOptInOnPreferencesPage = true;
$wgPopupsOptInDefaultState = '1';
$wgPopupsReferencePreviewsBetaFeature = false;
Page Previews do not appear. What to do?
Here's the wiki - wiki.mrowki dot ovh