Hello vincentd, have you already tried to use the hostname? CODESYS does not have the IP Address in their certificates. If the PLC cannot resolve a host name, no connection can be established via certificate. So you need to add the IP to the hosts file (after adding, delete the ARP cache if necessary, if IP is assigned via DHCP, the hosts file may have to be adjusted more often... ) or use the hostname if possible. IP address is just working for non-encrypted communication. regards