Signature verification failed & failed to find update binary

Using /e/OS Setup
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7

I have a FP3 and got the same problem when updating from Android 9 (dev) to Android 11. I solved it by first going through the installation of the FP3 stock ROM, as described here (steps 1-4), and then

  1. going again into the fastboot mode (first time was to flash the Fairphone Android 11 stock ROM; with Power+VolDown)
  2. again unlocking the phone with fastboot flashing unlock. After that in reboots automatically (I let it boot into the OS).
  3. rebooting into fastboot
  4. unzip IMG-e-1.6-r-20221129238947-stable-FP3.zip (the one I used) and change into the extracted directory.
  5. and finally execute the flash_FP3_factory.sh

After that I could simply reboot into my shiny /e/ Android 11.

This is basically the same procedure as to install the stock ROM.