AccelerateWP: End user can't activate "Site Optimization Module" - Found conflicting plugins: Unknown

Issue

An end user can't activate the "Site Optimization Module" from the AccelerateWP dashboard. Usually, this error message provides an excellent hint about the problem (a plugin needs to be disabled), however, in this case, the plugin is labeled as "unknown". 

Environment

• CloudLinux OS
• AccelerateWP
• WordPress

Solution

The solution is to look for leftover "advanced-cache.php" files within the user's wp-content directory. Removing/renaming the file allows enabling the module. 

Cause

A previous cache plugin probably causes the issue. The plugin might be removed/disabled, but leftover files can remain in the user's directories. This conflicts with the auto-generated files from the AccelerateWP suite, thus, creating the error.

Useful links

• Allowing the module for end-users
• AccelerateWP