1. Your database has SINGLE_USER access and it has been already opened. Make sure no one uses the database during backup or change the access mode. 
2. Name of the database has leading/trailing spaces or unprintable characters. Fix the database name by removing the unsupported characters.
3. Microsoft Volume Shadow Copy Service (VSS) and SQL Server VSS Writer Service are not running or unstable. Check system logs and solve the issue.
4. Lack of worker threads. Try to increase the Max Worker Threads configuration option of MS SQL Server.

Depending on the configuration of a SQL Server, available threads can be used if you create a snapshot backup of many databases at the same time. Microsoft has an article referring to the issue and its workaround:

• If you are running a 64-bit version of SQL Server, you can consider increasing the Max Worker Threads configuration option
• If you are running a 32-bit version of SQL Server, check the influence of increasing of Max Worker Threads on server performance

• For SQL Server 2008
• For SQL Server 2008 R2
• For SQL Server 2005
• For SQL Server 2012

***** In order to complete the backup successfully you will need to configure 5 Worker Threads for each SQL instance, for example: if you have 10 SQL instances (databases) you will need to configure 50  Worker Threads in the Max Worker Threads option. *****

Note: Online Backup (Datashield) is a legacy product only available to older Atera accounts. For newer backup solutions, check this page.