I spent about an our looking into this error. I had looked at my edmx and everything seemed normal for an auto generated concurrency field. I deleted the table, re-added, but still no luck.
I then looked at my model to see if anything had changed from another developer, and had noticed my concurrency field now had a [required] attribute. My thinking was that since I was validating the model before saving to the database, the null concurrency field was returning invalid since it was ‘required’. Deleting this attribute will allow the entity framework to save normally. Hope this helps!