Support for unsafe ciphers was removed as part of a security update in EcoStruxure IT Gateway version 1.8.0. NetBotz 4.x devices must now support 2048 bit certificates.
Devices that do not meet this requirement do not discover in Gateway 1.8.0 and newer. There is no indication of a certificate error when you run a new discovery.
When you update your Gateway from a previous version, NetBotz devices are listed as offline.
Install a new 2048 bit certificate on the NetBotz device. Self-signed automatically generated SSL certificates use 2048-bit encryption as of NetBotz version 4.5.4.