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.