.htaccess was not found (not caused by permalinks not being set)

Hi,

When I save the settings I get this error message: .htaccess was not found
I followed the link to the proposed solution, but I have already set the permalinks.

I think this is caused by having WordPress installed in a subdomain in a subfolder like this:

In WordPress
Home url: https://sub.domain.com/
Site url: https://sub.domain.com/wordpress

On the server
/domains/domain.com/public_html/sub/
.htaccess
index.php
/domains/domain.com/public_html/sub/wordpress
contains all WP files

FP Fastest cache does work correctly on the main domain where WP is also installed in a subfolder. I only get it when WP is installed in a subfolder in a subdomain.

(my development, test and demo sites are in sub domain of the main domain)

Thanks
JP