https://support.microsoft.com/en-us/topic/kb2520240-fix-permission-error-when-you-view-a-folder-by-using-the-viewmode-webpartlist-parameter-in-the-query-string-for-report-manager-in-sql-server-2008-r2-reporting-services-8d18d00f-a1fb-ec3a-e212-0e8a587b2390
KB2520240 - FIX: Permission error when you view a folder by using the "ViewMode=WebpartList"...