By default, MS SharePoint on Windows SBS does not have any MS SQL user with the sysadmin role.
You need to assign the sysadmin role for proper Acronis SharePoint operation.
Take the following steps to give any Windows user an MS SQL sysadmin role:
To fix the issue, allow MS SQL Server ports in Firewall settings
Acronis SharePoint Explorer: Extraction to functioning site or in SharePoint content migration package format fails with "Failed to create package file" or with empty message.
Change the "List View Threshold" option on SharePoint
Provide credentials of the user that has access to the network share on the Agent machine