Blank pages can be caused by a number of things, and often occur just after installing, upgrading, or customizing your store.  Any time you touch a PHP file, you must be sure to not introduce syntax problems, and you need to be sure that it uploaded correctly.

1. Did all your file-uploads work properly?

2. Have you created any syntax errors in your customizations?

A PHP syntax error will very often result in a blank screen, or a partially-blank screen.

When this happens, there is often also an entry in the server's errorlog, which you can often view via your hosting control panel area.

a) Turn on debug logging to store errors to a log file

Install the Debug Error-Logging Utility (note: this is ALREADY installed in v1.3.9x)

One way to identify what PHP errors are happening, especially if you have no access to server errorlogs, is to use the Debug Error-Logging Tool from our downloads area.   It's a simple install, and can expose the source of your problems fairly quickly.  Click {here} to download it. (note: it's already installed in v1.3.9x)

If you're dealing with blank admin pages, for the upload destination, substitute the /admin/includes/extra_configures folder instead of /includes/extra_configures where mentioned in the instructions.

The debug tool will create files in your /cache/ folder, named "myDebug-xxxxxx.log" (or "myDebug-adm-xxxxxxx.log" if they are from the admin side).
You'll want to read the contents of these files to see what the actual PHP errors are.
Then search this FAQ area for your error message and/or see the "Dealing with Error Messages" section, below ...

b) Attempt to show errors on-screen.

While the method above is far more reliable and preferable (since messages are never shown to customers), Another way to help identify *where* an error is happening is to show the errors on your browser. The drawback to this is that your customers can also see the errors, and search engines might catch them too ... which could be embarrassing.

To attempt to show PHP debug errors on-screen, create and upload a new file, like this:

Filename:  /includes/local/configure.php

<?php
  define('STRICT_ERROR_REPORTING', true);
?>

Then try to access your site again. You may see many warning messages on the screen. 

Dealing with the Error Messages

3. Common customization errors

• using ' or " marks inside define() statements, resulting in mismatched quotes
• removing or adding punctuation to define() statements, thus breaking the correct syntax.   A define() statement should look like this: define('CONSTANT_NAME', 'value here');
• missing semi-colons and periods, unbalanced parentheses, braces, brackets
• uploading files to the wrong places.  For more information about overrides, see the Overrides Chart and the Overrides FAQs