There can be several factors causing this kind of error. In my experience, this is usually due to plugins or theme conflicts that may happen after updating theme.

To troubleshoot for possible plugin conflict, you need to access your site data via cPanel File Manager or FTP or equivalent tool depending on your hosting or system.

Note: Create a backup of your site before doing the following.

1. Check for plugin conflict.
   • Via cPanel File Manager or FTP, go to YourWebsite/wp-content/
   • Rename the folder “plugins” to anything like “plugins_bkp”
   • Try refreshing the site if that solves the issue.
   • If the error is gone, this means the issue is caused by the plugin. Otherwise, go to number 2. Check for theme conflict.
   • To figure out the conflicting plugin, create a new folder with the name “plugins”
   • Individually copy each folder content in “plugins_bkp” to the new “plugins” folder you created.
   • Keep refreshing the site after each plugin copied to check if that specific plugin is causing the issue.
   • Once you find the problematic plugin, delete that and access your WP Dashboard to install a fresh copy of that plugin.
2. Check for theme conflict
   1. Via cPanel File Manager or FTP, go to YourWebsite/wp-content/themes/
   2. Delete the theme folder you are using for your site.
   3. Try refreshing the site if that solves the issue.
   4. If the error is gone, access your WP Dashboard to install a fresh copy of your theme.

If after installing fresh copies of your plugin or theme and the issue is not solved, contact support for the item.

I hope that helps.