It seems I got some faulty nginx configuration from an article on how to configure WordPress in a subdirectory.
Config Before
location @wp {
rewrite ^/blog(.*) /blog/index.php?q=$1;
}
location ^~ /blog {
root /home/user;
index index.php index.html index.htm;
try_files $uri $uri/ @wp;
location ~ \.php$ {
include fastcgi_params;
fastcgi_index index.php;
fastcgi_intercept_errors on;
# Deleted this line
fastcgi_split_path_info ^(/blog)(/.*)$;
fastcgi_pass unix:/var/run/php/php7.1-fpm.sock;
}
break;
}
Config After
location @wp {
rewrite ^/blog(.*) /blog/index.php?q=$1;
}
location ^~ /blog {
root /home/user;
index index.php index.html index.htm;
try_files $uri $uri/ @wp;
location ~ \.php$ {
include fastcgi_params;
# Added this line
fastcgi_param SCRIPT_FILENAME $request_filename;
fastcgi_index index.php;
fastcgi_intercept_errors on;
fastcgi_pass unix:/var/run/php/php7.1-fpm.sock;
}
break;
}
As mentioned in some comments, some PHP Server variables were incorrectly set due to a faulty nginx configuration causing issues when loading /wp-admin/customize.php
You can confirm your nginx configuration is faulty for this scenario if you visit /wp-admin/customize.php?wp_customize=on and it starts to work instead of throwing a fatal error.