The extension "Page previews" (aka "Previews", "Popups") is very old and haven't been updated for a long time. The last update: – (375d27b) 22:18, 23 September 2019
It is completely broken now. No preview is shown when you hover over a link.
This extension has "Reference Previews" too, but it is still broken on Miraheze.
Please update. I believe this should fix the issues. Thank you.