"Unfortunately, IMS service has stopped" after Marshmellow upgrade My Galaxy Note 4 was "upgraded" to Marshmallow today and I immediately started getting the popup notification "Unfortunately, IMS Service has stopped." This occurs as a popup about every two seconds, which effectively bricks the device. The problem occurs even if I boot into Safe Mode.

Things that have not helped:  


restarting after taking the battery out.  
wiping the cache for the Messages app.  
changing my default messaging application to Messages+.  
wiping the cache partition.  
checking for application updates.  
uninstalling Facebook Messenger.  


I found this thread which suggests turning off "rich communications," for an S5 Neo, but the Galaxy Note 4 doesn't appear to have that option anywhere.

At this point the only thing I can think of is a factory reset, which I'd very much like to avoid.
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