-128: Log backup is not possible because the log overwrite mode has been enabled 

Explanation:

You tried to save the log area, either by starting an interactive log backup or by attempting to activate the automatic log backup. This is not allowed in the current database state, because automatic overwrite mode for the log area is enabled. Enabling automatic log overwrite mode canceled the backup history, and the last data backup can no longer be used to fully restore the database instance. Therefore, it doesn't make any sense at present to back up the log.

User Action:

...

       1.      Disable automatic overwrite mode for the log area.

       2.      Create a complete data backup to begin the backup history again.

       3.      Repeat the failed command.