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You may need to increase memory PHP is able to use depending on how may modules or scripts you use on your site.
If you get a blank screen on any pages, you have two choices; increase your allocated memory for PHP or delete unused/misconfigured modules for the application.
This can be done in a number of places with the most likely being php.ini
Add for example:
memory_limit = 16M to your php.ini file
You will need to experiment with the value that is right for you depending on site needs. Some people find they need to set the memory to 24M or 32MB or higher (e.g. 96MB is recommended for a site with built-in image processing using ImageAPI GD).