Sophos for Microsoft SharePoint: installing to a server using a pre-existing remote database displays error

Issue
If you install Sophos for Microsoft SharePoint onto a server and choose to use a pre-existing remote database instance you get the error:

Setup was unable to configure the access permissions on the SQL Server instance Servername for user Username. Setup will now proceed but please contact Sophos Technical Support for information on how to perform this task manually.

Installation then continues, and completes successfully.

Technical Information
This issue can occur because the installer failed to configure access permissions for the user provided in the Sophos for Microsoft SharePoint installer.

We may fail to set access rights in certain scenarios, for example:

• when both computers (the Sophos for Microsoft SharePoint and the SQL Server one) are in a workgroup
• when one is in a workgroup and the other is in a domain

Sophos product
Sophos for Microsoft SharePoint

What to do

In this situation, you have two options:

• Make Windows authentication work.
  To do this, create a local user on the SQL Server computer with the same username and password as the one provided in the Sophos for Microsoft SharePoint installation. Login and database access permissions have to be added in SQL Server for this newly created user.
• Use SQL authentication.