Reasons for users getting “unauthorized access” error sporadically in SharePoint:
There are various reasons why you would get this error. Most often the issue lies with the permissions on the Site/Library/List. To isolate the cause check the following details
1. Please make sure that you have the permission for the item (list, library) or subsite etc that you are trying to access.
2. Please try to access the same List/Site/Subsite/Library multiple times and see if you have issues. If the issue is related to permissions then you will continue to get the error whenever you try to access the List/Site/Subsite/Library.
3. Contact the Site administrator and check with him/her
4. Check for the group membership
5. Check whether the user is present in control panel
6. Check whether the permission is broken as a result of not Inheriting the permission from parent site.
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