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Are you using Magento 1.4.1?
Please try with this:
A Magento Forum search suggested that the same thing happened with anyone using the Modern Theme. The fix is to change page.xml
<block type="core/profiler" output="toHtml"/> should be changed to:
<block type="core/profiler" output="toHtml" name="core_profiler"/>
When do you downloaded the theme?