I've made a test upgrade of our application to the most recent version of Umbraco (7.3) from the 7.3-RC. After I got it working I installed the newest Courier package (2.51.2 + and 2.51.2 - Hotfix) and tried to deploy all of the content from one instance (with all the data) to the other one with a fresh DB. However, every time I try that, I get an error message:

Application Error

Value cannot be null. Parameter name: compareRepository Error details

System.ArgumentNullException: Value cannot be null. Parameter name: compareRepository at Umbraco.Courier.Core.Tasks.PackagingTask..ctor(ExecutionContext ctx, Repository source, Repository destination, Repository compareRepository, String revisionAlias) at Umbraco.Courier.UI.Dialogs.CommitItem.RenderSummary(String target) at Umbraco.Courier.UI.Dialogs.CommitItem.oneSteptransfer(Object sender, EventArgs e) at System.Web.UI.WebControls.Button.OnClick(EventArgs e) at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

As a test, I've added a random image to the fresh instance and tried to deploy to the first one, still the same error.

Any reason, why this might be happening?

EDIT: Cloned the Umbraco, release-7.3.0 tag and tried to use Courier between those two instances, still got the same error.