If the scan server is too busy to accept new scan requests there could be a number of things causing this. Ultimately though the scan server can't keep up with the workload and hence you see the errors.
We would suggest performing a general review of your policies and environment.
Can you share some details about the appliance in use, so I can provide relevant KBs for you to check?
Was my reply helpful? If this information was helpful in any way, or answered your question, will you please select "Accept as Solution" in my reply, or give kudos as appropriate, so together we can help other members?