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No I'm pretty sure the 1.8V is connected. Should I try to disassemble the programmer and the 1.8V converter and assemble it again?
So you are using a 1.8V converter between the programmer and the chip?
Regarding the original issue: If you can not change any bytes, I would suspect the flash chip is write-protected. Check its datasheet, it is likely pin 3 is somehow pulled low (write-protect pin is usually active when low).
Regards,
Angel Pons