Diagnosis
The last repeat load is still running; no new requests are possilbe.
A repeat request is currently running for this DataSource.
You can request a new delta, only when the loading process is complete.

Procedure
Check the status of the repeat request in the Monitor.
If the load has been terminated, remove the error;
Set the request to red or green using the QM action, so that the nextdelta or a repeat can be requested.