Endless boot process after I turned on "QuickBoot"

Since today, my FP2 cannot boot anymore.
It seems a bit like this issue with FP1: Fairphone does not boot anymore

When I switch the phone on, I first get the black screen with the Fairphone and Android logo, then the blue screen with the Fairphone logo, then the light grey one with the “loading…” text.
After a few minutes, I get a message box for a very short time (cannot read it or take a photo). Then the procedure starts again with the blue screen and the light grey one.
I can only stop this by removing the battery.

During this the phone gets quite wam (CPU is running…).
I have fast boot mode enabled.

Same behaviour with 2 SIMs, 1 SIM, no SIM and no SD card.

Maybe related: In the recent days I experienced some (2-3) sudden reboots during a telephone call.