Operation terminated due to error (queue &1, target system &2)

What causes this issue?

When checking the queue for data transfer to APO at least one entry
with errors was found. This means that some of the data could not
successfully be sent to APO.

System Response

The integration model cannot be set to active.

How to fix this error?

Check the queues in the qRFC monitor (SMQ1 if you are using outbound
queues and SMQ2 in the target system if you are using inbound queues),
then remove the cause of the error. Restart the action.

Error message extract from SAP system. Copyright SAP SE.