Customize/Preview Not Working

Recently my Customize and Site Preview stopped working in the Unite theme and I lost some header and other settings. It seems to work fine in themes like MaxStore and Flexibility3 but not in most other themes I have installed. I’m not sure if something has gotten corrupt somewhere or I’ve lost a theme file. The functions.php and other files seem fine and look to be all there in the Unite theme folder. I’ve deleted the Unite theme and re-installed it and overwritten non-data wordpress directories wp-admin and wp-includes from another working wp install. Nothing seems to allow me to use customize, header, background or css from ‘appearance’. When clicking on one of those I see the browser tab say ‘customize:loading’ but stays there and only the boxes for ‘widgets’ and ‘active theme’ are visible but clicking them does nothing. Since the ‘customize’ feature opens and works with some themes, I suspect I have a corrupt theme file somewhere but seem unable to fix it. Do you have any suggestions for me? Thanks for your help.

It turned out that a plugin was having a conflict with the WP Appearance/Customize page. Deactivating the plugin solved the problem.