Can't access my sub-site dashboard - get an 'HTTP ERROR 500'

Hi I feel I have tried every possible option. I have a WP multisite withe one site on main domain and one site on subdomain withe mapped domain.
But I cant access dashboard on mapeddomain.com/wp-admin – original 2domain.maindomain.com
I have ben working intensively on the site for many days now and the error just came out of nothing.

This is part of my wp-config.php;

/* Multisite */
define( 'WP_ALLOW_MULTISITE', true );

define('SUNRISE', 'on');

define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', true);
define('DOMAIN_CURRENT_SITE', 'mymaindomain.com');
define('PATH_CURRENT_SITE', '/');
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);

And this is my .htaccess:

# BEGIN WordPress

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^wp-admin$ wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^(wp-(content|admin|includes).*) $1 [L]
RewriteRule ^(.*\.php)$ $1 [L]
RewriteRule . index.php [L]

# END WordPress

I hope some of you can help!