Unfortunately, there is no way to tell what exactly is causing this issue.

The best solution in this case is to change the template to the default one.

In order to reset your phpBB2 template to the default one, you should run two MySQL queries through the phpMyAdmin tool. Go to your cPanel and click on phpMyAdmin from the Databases box.

Once in phpMyAdmin, choose the phpBB2 database from the drop-down menu on the left. Then open the SQL tab and paste the following queries in the text field:

• UPDATE phpbb_config SET config_value = '1' WHERE config_name = 'default_style';
• UPDATE phpbb_config SET config_value = '1' WHERE config_name = 'override_user_style';

Click on Go. This will submit your query. Clear your browser's cache and open your forum. You will see that it is using the default template.