Before installing the Rise of the Necromancer expansion, everything worked fine. After downloading, my d3prefs.txt file just disappeared. The game won’t create a new one, even after trying quite a bit to fix it (scan and repair, full uninstall/reinstall (including battle.net), and even creating a d3prefs.txt file for it to save to).
EDIT: I took a look at my D3Debug file and it says;
“ERROR: Couldn’t create preferences directory. Path is locked or missing?”
I found the problem. I will leave this here for people who may have the same problem.
On Windows 10, with the most recent update, they added a security option called Controlled Folder Access. By default, this is turned on. You need to turn it off for Diablo 3 to make/edit a d3prefs.txt file.