I was able to boot into bootloader by long pressing power + volume up buttons on the Fairphone. Afterwards UBports installer was able to find the device and install UT OTA-8.
So everything runs fine again. Thanks so much

@kugiigi My Fairphone 4 runs on 20.04 stable channel.

@MrRoughwood Hi Jan,
I have had positive experience with the UBPorts installer and Ubuntu Touch 20.04 installation on my secondary device, a Fairphone with 128Gb in April 2025. So I was a little surprised to see that thread about a failed install a few days ago.
I was gonna wait until (hopefully end of September, thank you to all developers <3) the new Ubuntu Touch Version 24.04 gets released and install that on my main phone Fairphone 4 with 256Gb. I can get back to you once I'm done to tell you how it went.