As Lars Erik is saying, you can set the header. The reason for this is that OData v3 (or more correctly WCF Data Services 5.0) removed support for $format=json in the querystring since they were working on a new more compact json format that was released after 5.0, that do support $format=json in the querystring, and they wanted to avoid compatibility issues.
The latest build of Webnodes uses 5.0 of WCF DS. We have a new version in testing that uses 5.4, and that we've verified works with the site you're having the problem with, Helene.
Note that the new more compact json format in 5.4 might require some minor code changes. The alternative is to use the verbose header Lars Erik mentions to get exactly the same Json as in V2.
The new build using WCF DS 5.4 will be released later today.