Hi,
Thank you for the details. I have checked the back-end, and there is a fatal script error caused by another plugin (wp all import pro), see the reference: https://i.imgur.com/2oxTDiT.png
Because it blocks further script execution, the search backend cannot load properly. Their plugin author should consider not loading their plugin scripts on other plugin pages. This error is present on all the WordPress back-end, so you may find other issues as well. I strongly recommend updating or disabling that plugin.
