Moving to Support Forum from Discussion
The error message specifies the exact problem. Your hosting company limits the amount of memory that php can use (in this case, to 3MB), and the script has run over. acm_file.php has to do with phpBB's file based caching system, and building said cache is definitely one of the more resource intensive operations. Having said that, 3MB is quite a low limit.
You may be able to temporarily solve this problem by adding the following to the top of the file (just below <?php)
You should, however, contact your hosting company and ask them what to do about this in the long term. Incidentally, who are you hosted with?