And another thing: wouldn't it be better to say: "RightContent" => "HTMLText" when using a new HtmlEditorField?

Sorry for all the questions.. I'm only beginning here :-)

Are you aware if mod_security is enabled on your HTTP server? It has rather conservative settings. It's not uncommon for mod_security to identify a "Page Save" as an SQL injection attack and deny the query.

You could attempt to disable mod_security by appending the following lines in your .htaccess. This file resides in your SilverStripe root directory.

<IfModule mod_security.c>
SecFilterEngine off
</IfModule>

I'm experiencing the same "Error saving content" with the ArticlePage. Oddly enough, it saves and publishes the content until the browser cache is cleared, then it's gone. Weird. Any fixes for this?

same error here... the content is saved, but it keeps me saying "error saving content" my field is created as follows:

static $db = array(
'CostIncludes' => 'Text'
)
function getCMSFields() {
$fields->addFieldToTab('Root.Content.Main', new HtmlEditorField('CostIncludes','Cost includes'));
return $fields;
}

You are using a HtmlEditorField. Note that you have to use the FieldType of "HTMLText" instead of "Text" if you want to use a HTMLEditorField to edit it

dumb me... thanks for noticing the problem :)
now all is working perfect!