Try using:

link = Umbraco.NiceUrl(focusBox.GetPropertyValue("interntLink"));

instead :-)

Already tried that it jyst gives me an other error:
contains invalid arguments :P 

It could be the NiceUrl that is returning that one.

Try this for debugging:

link = focusBox.GetPropertyValue("interntLink");

Well, have also been there, and it also gives me an error:

The type 'object' can not be converted implicitly to 'string'

I'm really confused because - im pretty sure this have worked for me before :/ 

Try:

link = focusBox.GetPropertyValue("interntLink").ToString();

Ahh okay, but what should i do to the node ID i got then?

Try and check if it insert a node ID, or if it is missing somewhere. If there is one that is empty i could indicate that the HasValue() doesn't work.

Then you maybe could do something like this instead:

if (focusBox.HasValue("interntLink") && focusBox.GetPropertyValue("interntLink") != null && focusBox.GetPropertyValue("interntLink").ToString() != "") 
    {
        link = Umbraco.NiceUrl(int.Parse(focusBox.GetPropertyValue("interntLink").ToString()));
    }

NiceUrl parameter must be a int, so it should be like this

 link = Umbraco.NiceUrl(focusBox.GetPropertyValue<int>("interntLink"));

Hi Jeavon, worked perfect :D
I finally got it to work :D

Next time i'll post my Umbraco version as one of the first informations :D

Final code for curious people looks like this:

Umbraco version Umbraco v6.0.5

@inherits Umbraco.Web.Mvc.UmbracoTemplatePage
@using Eksponent.CropUp

@foreach(var focusBox in Model.Content.AncestorOrSelf(1).Descendants("FokusBoxRight")) 
{
    var mediaItem = Umbraco.TypedMedia(focusBox.GetPropertyValue("baggrundsbillede"));

    <div class="focusBoxRight" style="background: #000 url(@mediaItem.GetPropertyValue("umbracoFile")) no-repeat left top;">

        @if(focusBox.HasValue("overskrift"))
        {
            <h1>@focusBox.GetPropertyValue("overskrift")</h1>
        }


        @focusBox.GetPropertyValue("tekst")

        @{
            var link = "#";
            var linkTarget = "_self";

            if (focusBox.HasValue("interntLink")) 
            {
                link = Umbraco.NiceUrl(focusBox.GetPropertyValue<int>("interntLink")); 
            }

            else if(focusBox.HasValue("eksterntLink"))
            {
                link = focusBox.GetPropertyValue("eksterntLink").ToString();
            }

            else if(focusBox.HasValue("mediaLink"))
            {
                link = Umbraco.Media(focusBox.GetPropertyValue("mediaLink")).Url;
            }

            if (focusBox.Where("nytVindue = True"))
            {
                linkTarget = "_blank";
            }

         }

        <a class="readMore" href="@link" target="@linkTarget"><span><img src="/images/rld/btn_arrow_white.png" /></span>Læs mere</a>

    </div>
}

Good stuff and yes always a good idea to start forum posts including your Umbraco version :)

I would also use the type specifier here:

        else if(focusBox.HasValue("eksterntLink"))
        {
            link = focusBox.GetPropertyValue<string>("eksterntLink");
        }

is working on a reply...