My webhost is shared and the main website is set in the root directory. Five other websites are located as subdomains (see structure below):
The P3 plugin worked with subdomains but caused an 500 server error in the main domain:
/public_html/
/public_html/wp code of main domain
/public_html/wp-admin/ - maindomain wp code
/public_html/wp-content/ - maindomain wp code
/public_html/wp-content/plugins/ - maindomain wp code
/public_html/wp-content/themes/ - maindomain wp code
/public_html/wp-includes/ - maindomain wp code
/public_html/subdomain_name/ - subdomain wp code
/public_html/subdomain_name/wp_admin/ - subdomain wp code
/public_html/subdomain_name/wp_content/ - subdomain wp code
/public_html/subdomain_name/wp_admin/plugins/ - subdomain wp code
/public_html/subdomain_name/wp_admin/themes/ - subdomain wp code
/public_html/subdomain_name/wp_includes/ - subdomain wp code
etc