SQL Attachability Checks fail error "Cannot attach a database that was being restored" (4042125)

The nightly attachability checks fail with error

Microsoft.SqlServer.Management.Common.ExecutionFailureException: An exception occurred while executing a Transact-SQL statement or batch. ---> System.Data.SqlClient.SqlException: Cannot attach a database that was being restored.

The next conditions apply:

1. Attachability test is enabled to  'Use SQL Server on the protected machine'

2. Agent version is Rapid Recovery 6.x

SQL Log Shipping is enabled on the protected server

SQL Server Log shipping allows you to automatically send transaction log backups from a primary database on a primary server instance to one or more secondary databases on separate secondary server instances. The transaction log backups are applied to each of the secondary databases individually. An optional third server instance, known as the monitor server, records the history and status of backup and restore operations and, optionally, raises alerts if these operations fail to occur as scheduled.