This tutorial explains how to fix phpBB Unable to allocate memory for pool warnings and errors that show up on certain web server configurations.
VPS users with phpBB might come across an unusual error.
At the top of the page you will see a few lines of php warning with “Unable to allocate memory for pool …”
The cause of the problem is phpBB with APC Cache on.
How to fix Unable to allocate memory for pool warnings and errors
You might want to keep APC on for the rest of your websites so in order to disable it for phpBB only,
edit .htaccess file in the root of you phpBB and add the line:
php_flag apc.cache_by_default Off