my strange problem began with watching a video on Twitter with a Fairphone Open 18.04.1 on a Fairphone 2: suddenly the display froze, and I had to reboot.
This reboot, however, entered a endless boot loop. After the initial logo the Fairphone “startup bubbles” appeared two or three times, then it stuck, and rebooted again.
I searched the forum and tested all recommendations (taking out the cards, checking the slim case and the buttons, finally doing a factory reset), also this and this.
Then, I decided to try a reinstall and flashed the phone, first with Fairphone Open, then with Fairphone OS. In both cases the phone started into the endless boot loop.
After that I even tried Ubuntu Phone - with the same result: new startup logo (so I seemingly had installed Ubuntu successfully), but endless boot loop.
Now the strange part: With almost all hope lost, I tried Lineage OS according to this wiki. This means, I flashed the phone with a TWRP image, copied the Lineage zip to the sd card, and reinstalled using TWRP. And it worked!
So, unintentionally I now have a Lineage 14.1 running on my Fairphone, and I give it a try and test it.
But: I don’t understand what caused the problem, and why I cannot install Fairphone Open OS or Fairphone OS. Any suggestions?