like many others I stuck with unlocking my phone. Following the procedure which Jolla gave, my journey derailed after:
7.12 Enter the following command into the Windows terminal app (see 7.13 for an example about the actual command):
fastboot oem unlock 0x[inserting my received unlock code]
FAILED (remote: ‘unknown command’)
fastboot: error: Command failed
OKAY [ 1.716s] finished. total time: 1.720s
My upfront check for an unlockable bootloader said yes. The Developer options and USB debugging have been enabled whilst connected to an mobile network. Fastboot-Driver is installed as described and apparently working. The command
fastboot getvar all prompts all the device related data. (e.g.: product:XQ-AU52, (bootloader) unlocked:no, (bootloader) secure:yes)
I tried two different cables (one being the original Sony one) on two different computers (Win 10 with latest updates & Ubuntu 20.04) on all their USB ports. I used different versions of fastboot on Windows. I restarted the computer and the phone several times.
Trying and googling for five hours hasn’t brought any changes nor given any further ideas.
I remember it was already a f@#µng hassle on my Xperia X, but it finally worked. But this time I have to surrender and accept that I’m not computer savvy enough to install SFOS again.
My conclusion/advice is that it is better trying to install SFOS (free version) on a new phone first and then buy the license! Don’t be as dumb as I was