Cannot save css, header in editor. Menu etc work fine

Hello.
I’ve been looking at a lot of posts but did not find an answer.
In my admin panel I can perform almost every function, add pages, change the menu and de-install plug-ins but I cannot edit/save my CSS or anything in the editor. I have used advanced code editor, that just did not respond. When I moved back to the standard editor a message popped up:
“Service Temporarily Unavailable. The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.”

I have tried to “777” the css-file: same message.

Yesterday the site went public. I changed the settings on the page. It’s still in the same directory, just the URL was changed, both in WP and in STRATO’s admin page.

In my root folder htaccess looks like:

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]
</IfModule>

# END WordPress

in the /root/CMS folder it’s

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

# END WordPress

The site is on line, to the user it seems ok for now but I do need some tweaks. Anybody out there who can help? Thank you

(http://www.kwekerijkoning.nl Using a child theme of emphaino. I have uninstalled all plug-ins and re-installed the most important ones)