Cognos 8.4 Schedule cannot be Modified

It seems one cannot modify a schedule for a Cognos report. Any modifications to the schedule will cause the report to not be sent out. And get this, in the IBM Cognos Administration centre the reports are not listed as failed, but as succeeded. Only when opening the run history, am I confronted with this message: “CNC-SDS-0408 There are email messages waiting to be sent.”