Consider the following scenario in Dynamics SL 2015:
You have configured the Attachments feature with a document library on a SharePoint Online site. You try to access an existing attachment and receive the following error message:
System Message 333: Error reading file File: Object reference not set to an instance of an object
This message occurs when TLS 1.0 has been disabled. Microsoft is in the process of disabling TLS 1.0 and 1.1 in Office 365
More information is available at this link: https://docs.microsoft.com/en-us/microsoft-365/compliance/-1.0-and-1.1-deprecation-for-office-365?view=o365-worldwide
To work around this issue:
Reconfigure the Attachments to go to a network share
Upgrade to the latest Dynamics SL 2018 release.