When you have a Sharepoint library open within your browser, it creates a security key with the Sharepoint back end for access to the site content.
If you leave the browser open and do not access any site content for 15 minutes, the key times out and customers may get an access denied error similar to the below.
This is expected behaviour and the customer will need to refresh the browser, or close Sharepoint and open again to create a new key and access the content again.