Would it help if you delete all tasks from .archive file and re-create them again? I understand you can't stop the task from Scheduler. You can remove scheduler .archive file, where are all tasks stored from c:\Program Data\Rhinosoft\FTP Voyager, but that will remove ALL tasks.
Can you check your log files please? You can find them in UI or in c:\Program Data\Rhinosoft\FTP Voyager\Logs.