Login not working

Hi all.
I have a reoccuring issue.
I cannot login to the site.
I enter the correct credentials but the site simply asks for them again at the My Account page.
I have disabled all plugins and the issue remains.
I have tried renaming the theme folder but the site wont default to another theme, instead I get a white page with no content.

I suspect it’s something to do with the .htaccess file and the WP Rocket caching plugin. I think this because the cache resets every day and this is happening every day.
The issue was resolved by renaming the .htaccess file then renaming it back to the original name. But that method has not worked today.
I have manually cleared the cache folder via FTP and the issue remains.
Can anyone shed any light on this?

Thanks.
Will.

The page I need help with: [log in to see the link]