The error NS#125 "Incompatible firmware version" is raised because the firmware in the device does not match the firmware in the application file.
The FT6000 EVBs are loaded with firmware version 21 but there have been several releases of firmware 21 and they are not compatible. If you have installed the latest Izot NodeBuilder, you need to follow the instructions from the Izot NodeBuilder readme file , section 5 "Updating from IzoT Nodebuilder 4.3" to update the EVBs to the latest firmware. You should also recompile any application to make sure it is using the latest firmware.
Before updating the EVBs as detailed in the readme file, you might want to set your Izot router to use the repeater algorithm in OpenLNS CT. To do this, right-click the router -Properties-Advanced Properties-Router Type. Alternatively, follow the instructions from KB942