Fatal Error; no solution

Hello,

Keep getting the fatal error message:

Fatal Error: This should not happen but is most often caused by other plugins. Try first the option “Optimizer” in WP Staging->Settings and try again. If this does not help, enable wordpress debug mode to find out which plugin is causing this: 500 HTTP/2.0 500

Tested several workarounds including turning-off all plug-ins (except wp-staging and WP Staging Optimizer) from within wp-staging and wp itself. Same result with these tests.

Turned on debug and received these errors:

Warning: include(/home/seamusbe/public_html/wp-content/advanced-cache.php): failed to open stream: No such file or directory in /home/seamusbe/public_html/wp-settings.php on line 86

Warning: include(): Failed opening ‘/home/seamusbe/public_html/wp-content/advanced-cache.php’ for inclusion (include_path=’.:/usr/local/php56/pear’) in /home/seamusbe/public_html/wp-settings.php on line 86

Notice: has_cap was called with an argument that is deprecated since version 2.0.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /home/seamusbe/public_html/wp-includes/functions.php on line 3891

Warning: Cannot modify header information – headers already sent by (output started at /home/seamusbe/public_html/wp-settings.php:86) in /home/seamusbe/public_html/wp-includes/pluggable.php on line 1174

Any ideas how to solve?

Appreciate your help,

Seamus