I've currently been getting the same error message, but instead of finding an id of the MicroflowScheduler.Schedule entity, mine finds one from the ExcelExporterTemplateManager.MxConstraint entity. Which makes no sense as they're not related at all.
Have you been able to find a fix or cause for this? If not, does anyone else have any idea what might have caused this error?
Why would you even bother using a microflow from 2015 for something which is pretty straightforward? Retrieve ScheduledEventInformation in a microflow, perhaps make some kind of time threshold (older than..) and delete (in batches, depending on the quantity). Make it into a scheduledevent and let it run every week or so..