What operating system is used? What changes were made when this happened? You can check event logs for clues.
See the article below with a problem similar to yours, I believe it will help you better understand how to solve this scenario:
Go through the failover configuration process step-by-step and check the vendor classes option for two DHCP servers. According to my research, incompatible setting of the Vendor Class option on DHCP servers can cause this error. Check the vendor class of your two DHCP servers and add the missing class.
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