The proper phpBB2 functionality relays upon the domain name it is initially configured with. Once you change your domain name and start using it to access the phpBB2, you have to reflect the changes into the phpBB2 settings as well.

Because of the domain name change you will not be able to access the phpBB2 administration area. Therefore, you have to make the change through the phpBB2 database directly.

You should go to your cPanel > phpMyAdmin.

Once phpMyAdmin loads, from the drop-down menu on the left select the phpBB2 database. Once the page refreshes with the database contents, open the SQL tab at the top. In the text field paste the following SQL query:

UPDATE phpbb_config SET server_name = "newdomain.com" WHERE server_name = "olddomain.com";

You have to change "newdomain.com" with the new domain name of your account, and "olddomain.com" with the one you used to use with the account.

Once you are ready, click the GO button to execute the query. If everything goes without errors, the phpBB2 database will be updated and you will be able to use the forum with the new domain name.