I get "Unauthorized" when trying to access an organization

This is likely due to your OrganizationId and API key is not matching.
You need a "fresh" auth-token for each organization you want to access. The OrganizationId in the request urls need to be from the same organization as the API key used to get the Auth-token.

I'm trying to GET an invoice, but instead I get a list of invoices

You need to added Accept: application/json to your reuest header.
This is nessesary because this specific endpoint supports multible return formats.

My json model is correct, but I get an error saying 'The following models are missing: ....'

Check the encoding of the request body.

Why can't I make line breaks in offer/invoice/creditnote line descriptions?'

We do not support line breaks in the description, but each line can have a comment that will be displayed under the line. The comment can contain line breaks '\n'. Be aware that you cannot use html formatting in the texts, it's therefor not an option to use <br/>.

I use 'queryFilter' but it doesn't work

If the queryFilter is ignored, it might be because the backend, does not recognize the parameter, and since it's optional, it will be ignored. Check that your url is correctly formatted. Spaces should be represented as %20. Checkout the url encding guidelines http://www.w3schools.com/tags/ref_urlencode.asp

I've posted ledger items, but i can't find them in Dinero

A ledger item is a "Kassekladde linje". There a no vouchers created before they are booked, and ledger booking is not possible through the API. While un-booked, you can find them under "Regnskab">"Kassekladde".