Title: Error 500 on flushing cache Last modified: February 3, 2018 --- # Error 500 on flushing cache * [ahnabil](https://wordpress.org/support/users/ahnabil/) * (@ahnabil) * [8 years, 4 months ago](https://wordpress.org/support/topic/error-500-on-flushing-cache/) * i’m getting error 500 or save settings and flush cache * here’s the server log * ``` 2018/02/03 16:18:47 [error] 13460#13460: *176 FastCGI sent in stderr: "PHP message: PHP Fatal error: Uncaught Exception: unknown engine in /home/ahmed/web/example.com/public_html/wp-content/plugins/w3-total-cache/Cdnfsd_Core.php:48 Stack trace: #0 /home/ahmed/web/example.com/public_html/wp-content/plugins/w3-total-cache/Cdnfsd_CacheFlush.php(199): W3TC\Cdnfsd_Core->get_engine() #1 /home/ahmed/web/example.com/public_html/wp-includes/class-wp-hook.php(286): W3TC\Cdnfsd_CacheFlush::w3tc_flush_execute_delayed_operations(Array) #2 /home/ahmed/web/example.com/public_html/wp-includes/plugin.php(203): WP_Hook->apply_filters(Array, Array) #3 /home/ahmed/web/example.com/public_html/wp-content/plugins/w3-total-cache/CacheFlush_Locally.php(253): apply_filters('w3tc_flush_exec...', Array) #4 /home/ahmed/web/example.com/public_html/wp-content/plugins/w3-total-cache/CacheFlush.php(188): W3TC\CacheFlush_Locally->execute_delayed_operations() #5 /home/ahmed/web/example.com/public_html/wp-includes/class-wp-hook.php(284): W3TC\CacheFlush->execute_delayed_operations() #6 /... PHP message: PHP Fatal error: Uncaught Exception: unknown engine in /home/ahmed/web/example.com/public_html/wp-content/plugins/w3-total-cache/Cdnfsd_Core.php:48 Stack trace: #0 /home/ahmed/web/example.com/public_html/wp-content/plugins/w3-total-cache/Cdnfsd_CacheFlush.php(199): W3TC\Cdnfsd_Core->get_engine() #1 /home/ahmed/web/example.com/public_html/wp-includes/class-wp-hook.php(286): W3TC\Cdnfsd_CacheFlush::w3tc_flush_execute_delayed_operations(Array) #2 /home/ahmed/web/example.com/public_html/wp-includes/plugin.php(203): WP_Hook->apply_filters(Array, Array) #3 /home/ahmed/web/example.com/public_html/wp-content/plugins/w3-total-cache/CacheFlush_Locally.php(253): apply_filters('w3tc_flush_exec...', Array) #4 /home/ahmed/web/example.com/public_html/wp-content/plugins/w3-total-cache/CacheFlush.php(188): W3TC\CacheFlush_Locally->execute_delayed_operations() #5 /home/ahmed/web/example.com/public_html/wp-includes/class- ``` Viewing 4 replies - 1 through 4 (of 4 total) * Thread Starter [ahnabil](https://wordpress.org/support/users/ahnabil/) * (@ahnabil) * [8 years, 4 months ago](https://wordpress.org/support/topic/error-500-on-flushing-cache/#post-9933919) * ok it appears that this occurs only when trying to flush the page cache * Thread Starter [ahnabil](https://wordpress.org/support/users/ahnabil/) * (@ahnabil) * [8 years, 4 months ago](https://wordpress.org/support/topic/error-500-on-flushing-cache/#post-9936377) * it appears after enabling cloudflare, that the FSD CDN checkbox is checked, and greyed out, i think thats the issue * > [View post on imgur.com](https://imgur.com/a/DlCnT) * [ptblank](https://wordpress.org/support/users/ptblank/) * (@ptblank) * [8 years, 3 months ago](https://wordpress.org/support/topic/error-500-on-flushing-cache/#post-10079131) * Hey [@ahnabil](https://wordpress.org/support/users/ahnabil/) * I think your right. * Here’s an article by someone that agrees, with a fix. [https://techjourney.net/fix-wordpress-w3-total-cache-w3tc-uncaught-exception-unknown-engine-500-fatal-error/](https://techjourney.net/fix-wordpress-w3-total-cache-w3tc-uncaught-exception-unknown-engine-500-fatal-error/) * [Stefan Vasiljevic](https://wordpress.org/support/users/stefanue/) * (@stefanue) * [8 years, 2 months ago](https://wordpress.org/support/topic/error-500-on-flushing-cache/#post-10139611) * Hi There, * i managed to fix this by turning off the “FSD CDN” option from General Settings. * It was a really simple and quick fix. * Step 1: Right click on grayed-out checkbox -> Inspect * Step 2: Expand