Error log contained the following message:
Uncaught exception ‘Exception’ with message ‘unknown engine ‘ in /wp-content/plugins/w3-total-cache/Cdnfsd_Core.php
After some researching I found this post that gave me a hint on how to resolve this issue.
To correct the problem you need to edit
/wp-content/w3tc-config/master.php and change
"cdnfsd.enabled": "1" to
"cdnfsd.enabled": "0" and remove
"cloudflare" property if it is available.
It appears that this problem occurs if plug-in is switched from Premium to Community mode with Cloudflare enabled.
MaAnna Stephenson from Blog Aid writes why she yanked W3 Total Cache off her recommended plugins list. According to MaAnna W3 Total Cache had stability issues last year, still in Beta and doesn’t receive attention it needs from the lead developer.