NetBotz device does not add to ISXC 5.0 using the default user name and password.
InfraStruxure Central version 5.x
NetBotz any version
If you have created a user name and password combination under Server Administration Settings -> Netbotz Appliance Credentials for a specific range using a non-default password, ISXC will only use that combination for all devices in that range.
For instance, if a Netbotz device with an IP of 192.168.1.100 has the default netbotz/netbotz user name and password and you have also added apc/apc for the 192.168.*.* range, ISXC 5.0 will only attempt to communicate with anything in the 192.168.*.* range using the apc/apc user name and password including the device at 192.168.1.100.
To resolve this, you would either need to create a specific set of credentials for 192.168.1.100 or change it's user name and password to match the ones added for it's range. In the case above, you would change the Netbotz appliance user name and password to apc/apc.
This is not an issue with 6.x and higher.