This indicates that your database is corrupt or incomplete. This can happen in any number of ways. It could be another App that is poorly written or contains a bug. It can happen because the underlying file system is failing or corrupted.
For Strongbox specifically, if Strongbox is terminated or stops during a sync, it can leave the database partially transferred, with 0 bytes or less than its full size, making it corrupt and unusable, resulting in an "HmacBlocStream" error. To avoid this, ensure the sync completes; don't quit the app mid-sync.
While some storage providers handle premature termination gracefully and restore the previous good copy, others like SFTP servers may leave a partial, unreadable file. In such cases, retrieve a good database copy from Strongbox's local backups or elsewhere and manually copy it to the correct storage location to restore normal functionality.