As above with ubports-installer_0.8.4-beta_linux_x86_64.AppImage failed (latest version available) and I was unable to get the downloaded file SW_binaries_for_Xperia_AOSP_N_MR1_5.7_r1_v08_loire.zip / *.img accepted, I used ubports-installer_0.7.4-beta_linux_x86_64.AppImage, a previous version I still had on my computer and this worked.

After that I was able to reinstall with ubports-installer_0.8.4-beta_linux_x86_64.AppImage (latest version). This time it was not requested to manually download the image file. Installation worked.

Still there is the issue in ubports installer version 0.8.4 (ubports-installer_0.8.4-beta_linux_x86_64.AppImage) that installation of the image file failed before and why it is now working.

I made here a separate issue:
https://forums.ubports.com/topic/5540/update-versiom-609-failed-and-ubports-installer-fails

In case it helps anyone else I think I worked out why my phone stopped booting after updating. Originally it was running the /community/fredldotme/devel channel. I think I must have accidentally switched it to /arm64/hydris/devel in the phone settings playing around one day. In the phone they both show up as Development but in the installer you see the full path and can tell them apart.
Not sure if this was the problem, but I assume swapping between devel channels is not a good idea!

Thanks for the awesome work @fredldotme