I have two domains, one of them is subdomain of another:
xstarter.com : public_html/
automate-my-tasks.com : public_html/automate-my-tasks/

phpBB3 RC1 installed in the root of automate-my-tasks.com . After I am visiting automate-my-tasks.com and post new topic system redirect me to automate-my-tasks.xstarter.com . This URL is correct for system but not correct for users and google .

In the forum preferences (Genereal/Server configuration/Server settings) specified Domain name:automate-my-tasks.com . As I found in phpBB sources there are exist an macro functions (sorry, maybe this is incorrect definition) {$phpbb_root_path} and it replaced not by domain name from settings, but by host name.

I am sorry, I can not fix it myself, can not find place there this macro replaced by real value.