Xperia III stuck on step 8.7 after installation

I installed sailfish OS on a Sony Xperia III as per the instructions. After step 8.6, the phone just stays on with the blue LED on. I did get the correct output from the installer:

[...]
Flashing completed.

Remove the USB cable and bootup the device by pressing powerkey.

Pressing or holding the power key does not turn it off. Plugging it into a computer and running fastboot reboot does nothing.

It’s completely stuck in this form since yesterday (blue LED is on, screen is off, connecting to USB does nothing, power button does nothing). Any ideas?

Found a useful mention here:

Needed to hold down VolUp+Power for three seconds for a hard reboot. Maybe this should be mentioned in the installation guide?

Had the same issue and solution.
I wonder if we were supposed to close the terminal-fastboot session first and THEN disconnect the device? I disconnected the cable first and then pressed ‘any key’ (I was using windows for flashing) to close the command-window and was wondering if this was really the correct move after seeing the permanent blue LED afterwards. Did you do it the same way?

I flashed my 10 III four times up to now, and had this problem once or twice. The hard boot should solve this (did it in my case) - so perhaps this topic can be marked as solved with that solution…