If the theme Settings suddenly stopped saving, please try the follwing:
- Make sure that all files are uploaded completely.
- Empty your blog cache if you use any cache or CDN plugins also empty your browser cache.
- Make sure there is no any JS conflicts with any plugins. If the problem goes away while all plugins are inactive, then reactivate them one by one to determine which is causing the problem.
- If you use the Wordfence Security plugin please go to the Firewall settings page and uncheck the XSS option as it blocks the Ajax requests of the WordPress.