It is funny I was just discussing this very problem with the firmware developer just a few minutes before your post but on another device requiring secure class inclusion… The vision/zooz/monoprice 4 in 1 sensor.
Not sure how this is going to help you but I can at least share what I discussed.
This problem is a function of how large your network is and I discovered that when I am on a brand new zwave network, it includes without any issue. When I had about 70 devices I started needing 2 or 3 attempts in order for the secure class keys to exchange properly. Now with 143 nodes, I have near 0% chance to get it to include securely.
My suspicion from the behavior and the logs I have seen is that as the network gets larger, the vera UI response lag increases to the point of missing the time window the device is allowing to exchange the security key causing the device to timeout and depending on the device, to fail inclusion or to fall back to non secure inclusion which confuses the vera which then fails the configuration process.
My workaround has been to use openzwave and it is a pretty advanced operation requiring extraction of the security key from the vera, installation of openzwave and inclusion from it using the vera security key. I have had this problem for years and reported it to CS which lead to my workaround. Hopefully the next FW fixes it.