--- class.t3lib_tsfebeuserauth.php
+++ (NEW)
@@ -799,7 +799,7 @@
 			}
 
 				// override all settings with user TSconfig
-			if ($this->extAdminConfig['override.'][$pre.'.'][$val] && $val)	{
+			if (isset($this->extAdminConfig['override.'][$pre.'.'][$val]) && $val)	{
 				return $this->extAdminConfig['override.'][$pre.'.'][$val];
 			}
 			if ($this->extAdminConfig['override.'][$pre])	{