Solution 1
Might be that something made your settings file read only. You can normally fix this with deleting the settings.rgd file in the below user directory.
Solution 2
The settings are sync'd with the cloud, which may be having issues.
Go to https://store.steampowered.com/account/remotestorageapp/?appid=648800
Download the file ending with Player/Settings.rgd and replace your copy while steam and the game are closed (fully closed, check task manager if its running).
Then start steam by launching it with Run as Admin and start the game