This message is expected behavior if other tasks are currently accessing the files the task is attempting to access.
For example, an offload session will display this message if the Backup Job is running, as the backup job is given priority over the offload session.
However, there are situations in which the procedures to remove tracked action entries from the configuration database may not have been processed correctly, causing files to be marked as in use by a task that is no longer running. Often, this occurs when the Veeam Server is improperly shut down or loses power. If you suspect this has happened, you should create a case with Veeam Support.