On subdomains installations if the configuration php.ini file of PHP, have the open_basedir directive active, wp_w3all will not include the necessary phpBB config file, because files inclusions are restricted to the directory assigned by open_basedir in the server, and the phpBB config file inclusion from another directory (that are in this case domain or subdomain) won’t work.

To let wp_w3all include the config file between domains and subdomains on your server, you need to comment off the open_basedir in your php.ini in this way:
;open_basedir = …

and restart Apache

or in apache httpd.conf add something like:

<Directory /var/www/vhosts/domain.tld/httpdocs>
php_admin_value open_basedir none
</Directory>

On online servers you need to edit the php.ini file under your domain control panel:

“my host uses Odin/plesk hosting software and through that i found a phpsetting tab, allowing me to change open_basedir.
changed from:

{DOCROOT}{/}{:}{TMP}{/}

to

{WEBSPACEROOT}{/}{:}{TMP}{/}
and that solved it.”

NOTE about security implications: it is assumed here, that you know what it mean about the security, if the server’s folders aren’t all under the control of the same user.