skills ?

Did you check that your phone is possible to unlock?
https://www.youtube.com/watch?v=jWPY...youtu.be&t=34s

I had this same error message and I solved this buy enabling "allow OEM unlock"(= something like that - I can't remember exact name) from settings. After this I was able to unlock boot loader.

Of course you need your own unique unlock code but I guess that has been used.

Just to make a check list :

• The Jolla phone is updated to the latest 34.3.A.0.228 or .238 firmware (Important to do first because after unlocking the boot loader, OTA won't work as easily)
• Developer menu is activated
• USB debugging is on
• 3rd party application are authorizsed
• OEM unlocking is authorized
• You have registered your IMEI on Sony's website
• You've obtained a long hexadecimal code in return from Sony
• Your phone was first shutdown
• Afterwards (wait 10 seconds at least), your phone was turned on in Fastboot mode (hold "volume up" while plugging the USB cable)
• the Xperia is in Fastboot mode (no message on the screen, the LED is blue instead of green

At that moment you should run the fastboot command in your command shell (in cmd.exe) :
fastboot.exe oem unlock 0x<hexadecimal code>

Note that depending on how windows works, this might require admin rights (I have no idea, I use Linux. Maybe you'll get a UAC security prompt ? Or you need to "Run as..." cmd.exe to start the shell in admin mode ?)

there is no physical different between SDXC and SDHC card, they absolutely speak the same protocol (MMC).
the only difference is a logical/marketing one :
- SDHC are <= 32 GB and are sold pre-formatted with FAT32.
- SDXC are > 32 GB and are sold pre-formatted with exFAT (which is patented and cannot be provided by Jolla).

(that's different from plain SD cards which use a slightly older version of the protocol, that can only address card up to 1GB, or up to 4GB with a few hacks. the difference betwen SD and SDHC/SDXC is a bit similar to the introduction of LBA and LBA48 in harddisks).

this current situation is an entirely different problem : the card and the chipset are negociating a clock frequency to talk to each other (MMC uses a separate clock trace) - at least that's what it seems given the differences in dmesg between my card and the problematic card.

It currently seems that if the kernel is 3.10.84, this step fails (bug seems to be known in Ubuntu communities).
if the kernel is more recently (probably like the one packaged in Android 7.1.1 Nougat by Sony), this bug seems patched.
At least, that's what looks the most likely given the problems (card always works in stock android system and fails after Sailfish X flashing).

It just randomly happened the this card happens to be a 128GB one and also happens to have the frequency error.

But other people on TJC have successfully used a different models (the 256 Evo+ from Samsung).

Have you made this step described on Sony's "How to unlock bootloader" site?:

If not have you read this on Jolla's instruction site:
Reading instructions is mandatory to be successful!

social skills?

Sorry, too easy xD
But anyway, all they gotta do is to rtfm, nop ?
( especially with windows procedure )

I don't get "Delivered" status message after sending SMS. SIM is in the 2nd slot in F5122.
Do F5121 get this status or it's broken for everybody?

I met this issue ( on F5121, not F5122 ) after each reboot, network settings where lost ( roaming and "delivered" status settings where always disabled )
Got to use the trick for ofono config file to get them persistent between reboot. ( file owner and group radio:radio AFAIR )
Maybe it could help you

https://together.jolla.com/question/...-after-reboot/
OR :

Have you configured the Messages app to request a delivery report?