@bkos I also had sailfish on my device before and the same failed to recovery problem as you.
I managed to install it eventully a few days ago by installing twrp https://eu.dl.twrp.me/suzu/ with version 3.2.3-0 that that was mentioned in an earlier post.
The I got a bit further with it failing with "/cache/recovery/': remote couldn't create file: Is a directory" and then checked with adb shell and saw that the directory /cache/recovery was not there and created it.

then it got further and ended with

2019/10/28 21:30:45 Created ubuntu_command: /home/sonny/.cache/ubuntuimages/ubuntu_commands273504541
2019/10/28 21:30:45 Rebooting into recovery to flash

and then twrp came and i could not see those ubuntu commands.

Then I tried

fastboot flash oem SW_binaries_for_Xperia_AOSP_N_MR1_5.7_r1_v08_loire.img

again and then

ubuntu-device-flash touch --device=suzu --channel=16.04/community/fredldotme/devel --bootstrap

and this time it worked with another recovery screen.

I have no idea, this is my first time flashing ubports, but maybe its that there was something missing which twrp then provided and then flashing the recovery from the AOSP worked then.