To reproduce, set up a v5.2.0 Zabbix server and Zabbix Agent v5.0.5 with active checks.

 
I expect the client to connect to the server, retrieve a list of checks and start sending the data.

In reality, I see no data in the server and no error messages in its logs, and following error in the agent log:

root@kvmru05-19571:~# tail -2 /var/log/zabbix/zabbix_agent2.log
2020/10/30 14:57:32.762113 [102] cannot parse list of active checks from [46.101.10.22:10051]: refresh_unsupported tag is missing
2020/10/30 14:57:37.111443 [101] cannot parse list of active checks from [zabbix.terrty.net:10051]: refresh_unsupported tag is missing

I would expect agent v5.0 to continue working with server v5.2. The problem was likely introduced in ZBXNEXT-6220.