I found the solution on this blog posting: How to edit php.ini file to increase memory limit
Fatal error: Allowed memory size of 8388608 bytes exhausted at (null):0 (tried to allocate 184320 bytes) in /home/virtual/site3/fst/var/www/html/forum/cache/tpl_prosilver_overall_header.html.php on line 127
I managed to increase the memory in php.ini to overcome this, i will write here how i did it hoping that it may be useful for somebody.For those of you who use shared hosting the quick solution would be to add a line in .htaccess file located at your root folder.
php_value memory_limit 24M
here 24M is an example