I merged in some of my code to the examples/thread/dfu/client in the v4.1.0 SDK for Thread and Zigbee. After getting OTA DFU working, I moved onto some commissioning related stuff. I spent several days trying to figure out why I could not join my 52840DK to the Border Router. No matter what I did, I would get 'Join failed [Security]'. Wireshark shows failure in the DTLS handshake. I thought I had done something wrong, but turns out something in Nordic code has broken openthread.
For me this was very easy to reproduce. Simply take the examples/thread/dfu/client, and modify main.c to change ' .autocommissioning = true,' to ' .autocommissioning = false, .autostart_disable = true' so you can join a thread network rather than using built in hardcoded credentials.
Program the device (being sure to follow nordic instructions for mbr, bootloader, and this app), connect using the UART, and try to join a border router that is ready to accept the device.
> ifconfig up
Done
> joiner start TEST1234
Done
> Join failed [Security]
On a packet sniffer note that DTLS handshake fails.
I saw some other UNANSWERED devzone messages with a theme of nrf crypto breaking openthread DTLS. Not sure if that related, but one big difference between the thread/dfu/client and thread/cli example is the inclusion of nrf crypto related libraries. Also the thread/dfu/bootloader is part of the picture.
I am completely stuck at this point.