Various "Access Denied" error messages / lack of functionality on site

This time it's again not about development, but rather about multiple "unexplainable" behaviors on a SharePoint site which in fact have a simple reason. Supposed that you encounter any of the following symptoms on one / several of your SharePoint sites:

- "Access Denied" error message, although you have the necessary permissions (or even Site Collection Administrator / Farm Administrator privileges)
- "List does not exist" exception ("The page you selected contains a list that does not exist. It may have been deleted by another user."), although all the content is at the right place and no-one has made any modifications to it
- Several entries in the "Site Actions" menu are missing (such as Site settings, Edit Page, Create Site, Create Page), although you have the necessary permissions
- You do not see the possibility to edit any entries in the "List Templates" list
- ...

And here's a possible reason (quite frequently encountered actually) for all of this: the site collection has been set to read-only. Steps to perform during troubleshooting:

- check that the /_layouts/settings.aspx page is browsable on the site, and that you see all content available
- go to Central Administration - Application Management - Site Collection Quotas and Locks that the site is not set to "Read-only"
- if you cannot browse to that site collection, recycle the Application Pool which is associated to the IIS site hosting your web application / site collection

Hope this helps!