Hi abaper,
Aggregation period 4 indicates your aggregation process is based on hour of timestamp.It looks you have a overlapping aggregation consists of different posting dates,whereas your aggregation should be based on every hour of the transaction depending on one posting date.
Please check the following
1.Is your posting date the same as transaction date.
2.Do you have any open aggregates(That are not closed - with status 5)
3.Check if you have any scheduled jobs that didnt run on a hourly basis(if your intention is create o/b idocs too every one hour)
Alternately,if you really dont need to aggregate based on hour of timestamp, you can switch over to aggregation period as either "1" - Posting date or "11" - Posting date/Transaction currency.
Hope it helps
Best Regards,
Aram.