We just switched to a different ISP provider a few days ago. Since then, there have been several Bad Address entries in the DHCP server (Windows server 2012). After deleting them, they'll appear again in an hour. Their Mac addresses are not shown in the address lease.
I used the DHCPFind to check if there's any other DHCP server on the network. The result is attached.
How do I find out what device(s) is causing the issue? Or How to fix this issue?