there is a documented incompatibility between Admin Tools and Access Manager. The editor of Access Manager recommends the following workaround (http://www.pages-and-items.com/extensions/access-manager/faqs?faqitem=issues_admintools):
There is a small incompatability issue with Akeeba AdminTools, with certain settings you might get a white adminscreen in the backend in Access-Manager > menu/page access.
To fix this issue alter this file:
plugins/system/admintools/admintools/main.php line 630
$menu = JFactory::getApplication()->getMenu()->getItem($Itemid);
replace with
return array($option, $view);
Just make sure you keep a note of this alteration, so when you update Akeeba Admin Tools, you can do this fix again.
Is this issue also know to you and is a security issue if I alter the file mentioned?
Thanks,
Uwe