C5 automatically logs out

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After editing a page I click to edit another page and C5 logs me out. I get the page I want to edit but the top editing bar is gone. I go back to dashboard but I have to log back in????

Has anybody encountered this and if so how did you fix it?

I do have the following code in my template: <?php Loader::element('header_required'); ?>
thanks,
b

Sagrab