If you find the following error when inspecitng the server log after a failed entity publish with the error above.
07:29:16 - Error An unknown server error has occured System.NullReferenceException: Object reference not set to an instance of an object. at inRiver.Server.Repository.DataRepository.GetSpecificationAsHtml(Int32 specificationEntityId, Int32 entityId, CultureInfo ci) at inRiver.Server.Service.DataService.GetSpecificationAsHtml(Int32 specificationEntityId, Int32 entityId, CultureInfo ci)
...you most likely have CVL items without values. Simply run through all CVLs involved on the entity you published, and make sure they have both a key and a value. Missing values will result in this error - adding it makes everything work again.