The problem is usually caused by several things. The most possible one is because there is a page template using a master page based on itself. Can you review Design->Page Templates and check the column base on template and see if a template using a master page is based on itself. For example a template is using Page.master to create a template called MyTemplate and MyTemplate is based on a template called MyTemplate. This is causing a loop because MyTempalte is searching for MyTemplate which is also searching for MyTemplate and so on and this fills up all the server memory and the error is thrown.
Another cause is if you are using a custom navigation widget on some of the pages and in the code behind of the control IsNodeAccessible property is used. This is not working properly in Sitefinity 4.2 and above so it is best to remove it. It is also causing a loop.
Another cause is if using a widget that for example queries an image,document or video library and displays results on the frontend. Sitefinity API used for taking media content might be wrong and getting all images in the librari and then fitering them to show only a few (App.WorkWith().Images().Get().ToList and then filter with .Where() ).
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