I agent logs I see a lot of

  4879:20181114:193344.020 failed to accept an incoming connection: from 2001:bc8:XXXX TLS handshake set result code to 5:

and some

  4883:20181114:193344.159 active check data upload to [2001:bc8:XXXX:10051] started to fail ([connect] TCP successful, cannot establish TLS to [[2001:bc8:XXXX]:10051]: SSL_connect() I/O error: [0] Success)

and sometime I got

  4883:20181114:193352.564 active check data upload to [2001:bc8:XXXX:10051] is working again

On the server I got

  3340:20181114:193910.760 Zabbix agent item "proc.num[,,zomb]" on host "o4.domain" failed: first network error, wait for 15 seconds
  3376:20181114:193925.253 resuming Zabbix agent checks on host "o4.domain": connection restored

Network between server and agent is excelent - the same datacenter.

If I turnon debug logs on the server I got this while and error:

   389:20181114:190939.124 query [txnlev:0] [select h.proxy_hostid,h.ipmi_authtype,h.ipmi_privilege,h.ipmi_username,h.ipmi_password,h.tls_connect,h.tls_accept,h.tls_i
ssuer,h.tls_subject,h.tls_psk_identity,h.tls_psk from hosts h,items i where h.hostid=i.hostid and i.itemid=72691]
   389:20181114:190939.125 query [txnlev:0] [select hi.interfaceid,hi.type,hi.main,hi.useip,hi.ip,hi.dns,hi.port,hi.bulk from interface hi,items i where hi.hostid=i.h
ostid and i.itemid=72691]
   383:20181114:190939.126 End of zbx_tls_connect():FAIL error:'SSL_connect() I/O error: [0] Success'
   383:20181114:190939.126 End of get_value_agent():NETWORK_ERROR
   383:20181114:190939.126 Item [o4.domain:vfs.fs.size[/boot,pfree]] error: Get value from agent failed: TCP successful, cannot establish TLS to [[2001:bc8:XXXXX]:10050]: SSL_connect() I/O error: [0] Success
   383:20181114:190939.126 End of get_value():NETWORK_ERROR