This error and others like it often occur when updating from Drupal 6 to 7 and is caused by left-over settings from modules that did not clean up when they were uninstalled, and are not stored in the correct format. These will cause PHP errors.
A new module called Variable Check will check the database for errors and make it easy to spot the problem and delete the variable name that is causing the error message.
Step 1. Download and install Variable Check
Download the correct version from: http://drupal.org/project/variablecheck
Step 2. Install the Module
Go to Modules > Install new module and use the installation facility to install the module
Browse to find the module and install it by clicking Install.
Step 3. Activate the module.
Go to Modules and Other, and check the Enabled Box.
Step 4. Run the report
Go to Modules > Check System Variables
You will see the variables that are causing you a problem.
Step 5. Delete the variables
Check the boxes and click delete.
Confirm the delete.
