Getting "E: Unable to find bootable image" when flashing ncs-inter lesson 9 exercise 3 solution

[Oli]

Anyone knows why I'm getting this result? I'm building the application using sysbuild and flashing normally on nrf52840dk. I tried both following all the steps in the lesson as well as building/flashing the solution itself with the same result. 

*** Booting MCUboot v2.1.0-dev-12e5ee106034 ***
*** Using nRF Connect SDK v2.9.1-60d0d6c8d42d ***
*** Using Zephyr OS v3.7.99-ca954a6216c9 ***
I: Starting bootloader
I: Primary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
I: Secondary image: magic=unset, swap_type=0x1, copy_done=0x3, image_ok=0x3
I: Boot source: none
W: Failed reading image headers; Image=0
E: Unable to find bootable image

[Oli]

I believe I got the exercise working by switching sdk/tools to 2.9.0 instead of 2.9.1.