Where are force click preferences stored?
All your settings are stored in something called the defaults system, which at its core is a bunch of .plist files in your Library folder: ~/Library/Preferences/whatever.plist.

You can actually change them with the defaults command in Terminal, although it's generally a good idea to quit the relevant app first. Changing system settings will probably not take effect until you log out. For more information, run man defaults in Terminal.

Try this command:

defaults read -g | grep trackpad


This should show your trackpad-related settings. I don't have a Force Touch trackpad, but here's  what I get:

"com.apple.trackpad.forceClick" = 1;
"com.apple.trackpad.scaling" = "0.6875";
"com.apple.trackpad.scrolling" = "0.3125";




Now for some troubleshooting.

Before going any further, it's a good idea to duplicate your global preferences file, ~/Library/Preferences/.GlobalPreferences.plist, in case you lose some settings you want to keep.

To see if one of these preferences is the problem, try doing this with each preference in turn that you got from the previous command:

defaults delete -g com.apple.trackpad.forceClick


replacing with the proper one, of course. You'll probably have to log out and back in to see the changes.

(An important note: Terminal commands generally don't ask for confirmation before doing things. They assume you know what you're doing, which is all well and good until you don't, or make a typo.)