@jdesignz, can you please explain that solution? I can do it, but I'm not very code-savvy so I'm wondering if it's a good solution for anybody.
I'm having the same error 500, though it only comes up when I try to go to the "CUSTOMIZE" page on my theme (everything else works great), and when I try the same thing on my localhost version of my website, everything works - no error. Same versions for all of the themes and plugins.
I tried re-installing WordPress on my official site, and there's no difference.
Thanks!