Morgen Helmut,
if I have a look at the report info I read:
Report for processing inbound IDocs not passed to the application immediately.
This report forwards all IDocs with:
Status 64 "ready to be passed to application"
Status 66 "IDoc is waiting for predecessor IDoc (serialization)
In your case it can only be Status 66 and thus the cause is serialization.
regards,
Job