I can't clear the cache on the site

I’m getting this error when I try to delete my cache using Hummingbird Pro:

“There was an error during the cache purge. Check folder permissions are 755 for /wp-content/wphb-cache or delete directory manually”

However the folder that needs the permissions changed to 755 already has it changed to 755. I checked FTP and the folder is set to be 755.

The problem I’m having regarding the cache is that on my homepage https://the****ip.com/ under TGP Blog (bottom left), when you are logged in to the site you see the latest post properly, but if I look at the site when not logged in, the last post is a cached one, even having the option to “Clear full cache when post/page is updated” enabled.

The only way to fix this is by deleting the /wp-content/wphb-cache, which I did. But since the site is live and there are regular updates on it.

I have currently uninstalled the plugin on the site not to get the issue again, but I would like to know if there is any setting or fix that I could apply to the site to prevent this to happen again so I can continue using the plugin.