Can't unbrick my phone running UT2K

[yvmuell]

So your phone has Ubuntu installed or your PC, or what is UT2K?

and to be sure you are trying this?

Fairphone 5 UNBRICK w/ Bootloader locked - Android 14(+?) Bootloader Only

In fact, when received, the bootloader version was still FP5.UT2E.B.078.20241105, which corresponds to Fairphone OS Android 14. As if /e/OS installation was failed… I flashed this version, to check the phone was functional under Fairphone OS. Then unlocked bootloader, to flash Official /e/OS (Android 13). Since then, I kept it unlocked (because I understand from what you say that allow-flashing is not available with Android 13…)

And just for the records, fastboot and adb are 2 different things, your phone was booted to bootloader when you tried?