markarian on "[Plugin: WP Fastest Cache] 500 server error on 1 out of 3 sites"

This plugin is great for 2 of my sites, but the 3rd one gives a 500 error.
Same settings, similar hosting and configuration.
Get a couple of error messages about permission need to be 755 on two directories cache and cache/all which they already are.
Thanks

https://wordpress.org/plugins/wp-fastest-cache/