Site redirects to wrong url when saving settings

Question

I have a subdomain:

https://blog.example.com/

I forcibly redirects to subdirectory:

https://www.example.com/blog

By changing site URL and some RewriteRule on .htaccess.

Now the website and admin works fine except a small issue like when I tried to click save button on settings page (eg: Setting > Writing ) the site redirects to main domain:

https://www.example.com/wp-admin/options-writing.php?settings-updated=true

/blog is missing in the URL and end up in to main website 404 page.

My .htaccess file looks like :

    # BEGIN WordPress

<IfModule mod_rewrite.c>
RewriteEngine On

RewriteCond %{HTTP:X-Forwarded-Host}i ^example\.com
RewriteCond %{HTTP_HOST} ^blog\.example\.com$
RewriteRule ^/?(.*)$ https://www.example.com/blog/$1 [L,R=301,NC]

RewriteCond %{HTTP:X-Forwarded-Server}i !^www\.example\.com
RewriteCond %{HTTP_HOST} ^blog\.example\.com$
RewriteRule ^/?[0-9]{4}/[0-9]{2}/(.*)$ https://www.example.com/blog/$1 [L,R=301,NC]

RewriteCond %{HTTP:X-Forwarded-Server}i !^www\.example\.com
RewriteCond %{HTTP_HOST} ^blog\.example\.com$
RewriteRule ^/?(.*)$ https://www.example.com/blog/$1 [L,R=301,NC]

RewriteCond %{HTTP:X-Forwarded-Proto} !https
RewriteCond %{HTTPS} off
RewriteRule ^/?(.*)$ https://www.example.com/blog/$1 [L,R=301,NC]


RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

# END WordPress

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /blog/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog/index.php [L]
</IfModule>

# END WordPress

When click save button, console have an error message:

/wp-admin/options-general.php?settings-updated=true:1 GET https://www.example.com/wp-admin/options-general.php?settings-updated=true 404 ()

Can anybody help to figure this out?

Updated working .htaccess (according to 1st answer) :

<IfModule mod_rewrite.c>
RewriteEngine On

RewriteBase /blog/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /blog/index.php [L]

RewriteCond %{HTTP:X-Forwarded-Proto} !https
RewriteCond %{HTTPS} off
RewriteRule ^/?(.*)$ https://www.example.com/blog/$1 [L,R=301,NC]
</IfModule>

My wordpress address and site address are:

WordPress Address (URL) =   /blog
Site Address (URL) = https://www.example.com/blog

Answer 1

From looking at your .htaccess, it looks like your has too many rules overlapping each other. Do the following instead:

1. Keep a backup of your current .htaccess if you need to go back to it
2. Delete your .htaccess form the server, WordPress should automatically create a fresh one for you
3. Open your new .htaccess file and add the following between the <IfModule mod_rewrite.c> tag:

.

RewriteCond %{HTTP_HOST} ^blog\.example\.com$
RewriteRule ^/?(.*)$ https://www.example.com/blog/$1 [L,R=301,NC]

Your .htaccess should then look like the following:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
# Custom rewrite
RewriteCond %{HTTP_HOST} ^blog\.example\.com$
RewriteRule ^/?(.*)$ https://www.example.com/blog/$1 [L,R=301,NC]
</IfModule>
# END WordPress