I have CC'ed Martin and Yevgeni, they might be able to help out. Could you find a way to provide Engine logs to us so it is easier to debug this?

Can you also give a little more detail? For example, what are the MAC addresses that are claimed to be in use? What is the MAC address range of the default and extra MAC pool you created (do you think the error messages are valid, or do they seem wrong)?

If you need assistance finding out about the default MAC pool range, please reply to this thread, I think I might be able to show you a REST API command to query the default MAC pool details.