Sophos for Microsoft SharePoint: installation fails at credentials screen

Issue
Installation of Sophos for Microsoft SharePoint fails at the credentials screen with the message:

Setup was unable to access Microsoft SharePoint using the specified account.
Please ensure that the specified account is a local administrator and has full access rights to the entire SharePoint store.
Do you want to continue anyway?

Sophos product and version
Sophos for Microsoft SharePoint

What to do

There are two possible causes for this error:

• The account that you provided has insufficient access rights. It requires full access rights to the entire SharePoint store. Refer to the knowledgebase article, Sophos for Microsoft SharePoint: permissions required by services account.
• User Access Control (UAC) is enabled. ,You will always get this message if UAC is enabled, even if you have the correct rights. This is because UAC prevents the installer checking the SharePoint permissions of the user. In this case you can either confirm to continue anyway, or disable User Access Control to enable the installer to perform the checks.