We used to have the following setup and courier worked nicely. BTW we are using latest courier express.

Live server - 4.11.7 staging server - 4.11.7 dev server - 4.11.7

We are currently going through an upgrade process so currently we have

live server - 4.11.7 staging - 4.11.10 dev - 6.1.6

Now when we courier from staging to live all works fine. However courier from staging to dev does not we get error in log

"Can't import provider 'Umbraco.Courier.Persistence.V4.NHibernate.NHibernateProvider' id: e0474ca8-e73b-11df-9492-0800200c9a66: System.NullReferenceException: Object reference not set to an instance of an object."

Our staging server is effectively the master we do all content edits there and then courier to live. The dev we use for any code changes we test on there then courier to staging and then to live.

Eventually all 3 servers will be on 6.1.6 however there is mobile functionality being developed currently and after that has been deployed the upgrade will be done. So is 6.1.6 and 4.11.10 incompatible when using courier?

Regards

Ismail