Since we upgraded to the zabbix server version Zabbix 3.4.0 zabbix server notifies "Host is not reachable" although the agent is reachable:

root@monitor01 # telnet liamir01a.publicis.de 10050
Trying 10.250.72.21...
Connected to liamir01a.publicis.de.
Escape character is '^]'.

i upgraded the agent also to Zabbix 3.4.0 (revision 71467). This didn't fix the issue. Here is the agent config:

AllowRoot=0
BufferSend=5
BufferSize=100
DebugLevel=3
EnableRemoteCommands=1
HostnameItem=system.run[hostname -f]
Include=/etc/zabbix/zabbix_agentd.d/*.conf
ListenIP=0.0.0.0
ListenPort=10050
LogFile=/var/log/zabbix/zabbix_agentd.log
LogFileSize=1024
LogRemoteCommands=0
MaxLinesPerSecond=100
PidFile=/var/run/zabbix/zabbix_agentd.pid
RefreshActiveChecks=120
Server=10.200.126.51
StartAgents=1
Timeout=30
UnsafeUserParameters=0