Not Acceptable error at wordpress

When I submit a form which contain Facebook & instagram image URL
An error occurred

Not Acceptable

An appropriate representation of the requested resource /poster/corp-save-image/ could not be found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

My .htaccess code is

# BEGIN WordPress

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /poster/
RewriteRule ^index.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /poster/index.php [L]
</IfModule>
<IfModule mod_security.c>
  SecFilterEngine Off
  SecFilterScanPOST Off
</IfModule>

# END WordPress