Htaccess proxy redirect wordpress error

Solution1:

Try to change the paths of images (the URLs) in advanced layer slider and icons URLs from relative to absolute ones (as described in RFC 2396), then you can use the [P] flag in .htaccess again.
Often plugins uses relative paths when outputting content, but this can cause such type of troubles.

Solution2:

This following code takes care of permalinks in WorPress and it is recommended to keep the beginning and ending WordPress lines so that WordPress is able to locate it and modify it as needed. You may already have all this and are just not displaying it. I do not know.

# BEGIN WordPress

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

# END WordPress
In the other section you probably need to remove the trailing / after folder2 like so:

RewriteEngine On
RewriteCond %{REQUEST_URI} ^/folder
RewriteRule ^(.*)$ http://www.domainB.com/folder1/folder2 [P]