"Unfortunately, IMS service has stopped" after Marshmellow upgrade
Settings → Applications → Settings Button near top right corner of your screen (do not scroll down and find settings under the list of apps) → Default Apps → SMS App → choose another option to eliminate pop ups and identify which app is not working properly after the upgrade. After you delete or update the app causing the error, you can go back to your previous SMS choice if you wish.

You are not having a Marshmallow problem. The problem is with a new App not that has not properly been updated from Marshmallow support