Sony F5121: Forgotten Device Password, Bootloader Locked

I have forgotten the device encryption password for my F5121 (boot loader locked). Is there a way to reset the device so that I can use it again (the data it contains does not bother me)?

I've already tried a combination with the Volumne + Up / Down power button, but I do not see any options appear.

In addition, unlocking the boot loader does not seem to work (I would have probably had to activate the OEM unlock in the Android settings before), it says:

FAILED (remote: "Unauthorized command")

Same mistake when trying to flash TWRP.