I have been handed a site to work on. It's on 8.5.1. The site was working. However, I cannot make changes on certain configs. For example, the dialog boxes for the views do not open anymore. I keep getting the following js error under the console.
I do not want to revert the database back to a working condition since the client would lose their data.
Update: The dialog box does work very very random.
www.yourwebsite.com/update.php
And where would I add the jQuery UI?
@Maria might aswell update Drupal Core to the latest 8.6.1, If you have any missing core files, this will fix your issue. Remember to run update.php after.