All of the OSSEC services start at the same time and occasionally, the service that opens the socket for communication is not finished opening before the other services try to communicate which will result in this notice.
Cannot find '/queue/db/wdb'
It is usually harmless, but if you see this error and find that your agents are having trouble connecting to the HUB run:
awp -s -f
This will start the services in an order that will not produce this error.