Blank screen when empty/purge cache

I had same experience. This is dump from error_log:

[27-Feb-2018 14:56:23 UTC] PHP Fatal error:  Uncaught exception 'Exception' with message 'unknown engine ' in /home/vhost07321/public_html/wp-content/plugins/w3-total-cache/Cdnfsd_Core.php:48
Stack trace:
#0 /home/vhost07321/public_html/wp-content/plugins/w3-total-cache/Cdnfsd_CacheFlush.php(199): W3TC\Cdnfsd_Core->get_engine()
#1 [internal function]: W3TC\Cdnfsd_CacheFlush::w3tc_flush_execute_delayed_operations(Array)
#2 /home/vhost07321/public_html/wp-includes/class-wp-hook.php(286): call_user_func_array(Array, Array)
#3 /home/vhost07321/public_html/wp-includes/plugin.php(203): WP_Hook->apply_filters(Array, Array)
#4 /home/vhost07321/public_html/wp-content/plugins/w3-total-cache/CacheFlush_Locally.php(253): apply_filters('w3tc_flush_exec...', Array)
#5 /home/vhost07321/public_html/wp-content/plugins/w3-total-cache/CacheFlush.php(188): W3TC\CacheFlush_Locally->execute_delayed_operations()
#6 [internal function]: W3TC\CacheFlush->execute_delayed_operations()
#7 /home/vhost07321/public_html/wp-includes/class-wp-hook.php(284): call_user_func_array(Array, Array)
 in /home/vhost07321/public_html/wp-content/plugins/w3-total-cache/Cdnfsd_Core.php on line 48

Because on Performance -> General Settings -> CDN -> FST CDN Type -> Cloudflare is chosen from dropdown but control is disabled, below the line:

$engine = $c->get_string( 'cdnfsd.engine' );

I added following code to file wp-content/plugins/w3-total-cache/Cdnfsd_Core.php

			if ( empty( $engine ) ) {
				$engine = 'cloudflare';
			}

I hope W3TC will release proper fix for this.

Cheers,
Aleksandar

I had no choice and moved to supercache plugin, the only problem with this one is that it doesn’t preload pages on mobile only on desktop

Aleksandar, thank you! I would have never figured that one out in a million years!

If your still having issues you can also try disabling(setting to 0) the fsd cdn option in the master.php under wp-content/w3tc-config.

https://techjourney.net/fix-wordpress-w3-total-cache-w3tc-uncaught-exception-unknown-engine-500-fatal-error/

“Hello, First of all, sorry for the late response and the issue you are having
Is the issue resolved?
Please make sure to update W3TC to the latest version as a lot of fixes have been applied including the fix for the issue you are having.
If the issue is resolved after the update, please give us a feedback and we can close this topic.

Best Regards,

Marko”

It still happen to me.

Me too, It seems to clear the cache but the redirect to https://mysite.example.com/wp-admin/admin.php?w3tc_note=flush_pgcache

I’m also still having this issue. Using the most recent version of WordPress and the plugin.