One of my page types doesn´t show the meta data (keywords, description) in the frontend after I customized the URL interprtation:

Director::addRules(100, array(
    'view/$ID/$Name' => 'SomePage_Controller'
));

Is this a bug? What can I do about it? As soon as I remove this rule, the meta keywords etc will show in the xhtml code.

MetaTags are part of SiteTree. So if you don't have a SiteTree (or subclass), the method is not available.

Does SomePage_Controller return a Page somewhere?

Yes it does, it´s a child of Page.

Presumably, you are returning a DataObject through that page.

add the following to your DataObject class

function MetaTags($includeTitle){
return SiteTree::MetaTags($includeTitle);
}

Because you are accessing the controller directly, there is no Page object associated with the request and therefore no existing Meta Tags to use. The above code will allow you to return the Meta Tag values you have assigned to your DataObject through your view function.

I am showing a DataObject on that page by urlParams['ID']. I tried SiteTree::MetaTags() as a return value but it didn´t work. MetaTags in not a static function.
Thanks for Your hint anyways. I implemented a function MetaTags() in the SomePage_Controller and now it works. This isn´t a clean solution i suppose.