Read the article below in case it is a config issue, like lack of memory
Settings Won’t Save from Customizer
Once you are sure you have the right amount of memory and config settings, you can also try different browsers. IE and Edge for example are significantly slower at loading the customizer.
You can also try deactivating all non weaver plugins to see if one is causing the system to choke.
In case of emergency, you can turn off the Xtreme Customizer, so it goes back to the default WordPress in Advanced options > Admin options > Disable Weaver Xtreme Customizer.
In that mode you should be able to access WordPress header background… customizer options, and you can still set Xtreme Option in the Legacy Admin interface (Weaver Xtreme Admin > Main Pages) until you figure out the issue