This message is expected behavior if other other tasks are currently accessing the files that 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 was improperly shutdown or lost power. If you suspect that this has happened, you are advised to create a case with Veeam Support.