Fix for "Fatal error: Uncaught exception 'Exception' with message 'unknown engine'"

[nigrosimone]

From official forum https://wordpress.org/support/topic/fatal-error-2749/

Fatal error: Uncaught exception ‘Exception’ with message ‘unknown engine ‘ in /home/swansph/public_html/wp-content/plugins/w3-total-cache/Cdnfsd_Core.php:48 Stack trace: #0 /home/swansph/public_html/wp-content/plugins/w3-total-cache/Cdnfsd_CacheFlush.php(222): W3TC\Cdnfsd_Core->get_engine() #1 [internal function]: W3TC\Cdnfsd_CacheFlush::w3tc_flush_execute_delayed_operations(Array) #2 /home/swansph/public_html/wp-includes/class-wp-hook.php(286): call_user_func_array(Array, Array) #3 /home/swansph/public_html/wp-includes/plugin.php(203): WP_Hook->apply_filters(Array, Array) #4 /home/swansph/public_html/wp-content/plugins/w3-total-cache/CacheFlush_Locally.php(253): apply_filters(‘w3tc_flush_exec…’, Array) #5 /home/swansph/public_html/wp-content/plugins/w3-total-cache/CacheFlush.php(188): W3TC\CacheFlush_Locally->execute_delayed_operations() #6 /home/swansph/public_html/wp-content/plugins/w3-total-cache/CacheFlush.php(194): W3TC\CacheFlush->execute_delayed_operations() #7 [internal function]: W3TC\CacheFlush->execute_delay in /home/swansph/public_html/wp-content/plugins/w3-total-cache/Cdnfsd_Core.php on line 48