[PC] game difficulty has been constantly changed from nightmare to casual

Hi, I've been playing an Elf rogue on nightmare difficulty. However, I found my game difficulty has been somehow changed to casual when I checked my game options. Further, the difficulty option is locked. I managed to reset the game options (so the difficulty changes to normal) and then set it as nightmare again, but it has again been changed to casual if I reload the game or went to Haven (don't know if it will also change by just going to another place).

Has anyone met a similar problem? How should I fix this?