What is the hotfix you are currently on? There looks to be a bug related to this exact issue that was hotfixed in Kentico 9.0.4:
Scheduler - Error in the scheduled task list when using the external scheduling serviceAn error occurred when loading the list of scheduled tasks if any task had its "Period" property set to "Once" and was configured to be processed by the external scheduling service.
I would recommend applying the latest hotfix and checking to see if this resolves your issue.