Not sure if it helps, but in the Fairphone forum there were some instructions how to sign TWRP images so that they can be used for a one off boot (without installing).
It's basically this: https://z3ntu.github.io/2017/06/16/Signing-boot-images.html
I tried to fiddle around with the ubports-installer sources and added some debug output to see the exact command that fails and gives the "FAILED (remote: bootimage: incomplete or not signed)" error:
fastboot boot "~/.cache/ubports/images/FP2/boot.img"
If I sign the image according to the link above and use that instead, I don't get the error.
Edit: it seems this is already being worked on https://github.com/ubports/ubports-installer/issues/184 and the problem is not with the file being signed or not.