I know nothing about DEF-equipped tractors, but I have a 2015 Mercedes diesel Sprinter that has all that stuff. Seems like the biggest issue with DEF systems are the two NOx sensors. If either of them quits, a Sprinter will display a DEF code. There have been numerous updates on those sensors, just in the past few years. You may have to go to a dealer to connect with their diagnostic software.
How are your engine temps? Dose your tractor run cool all the time?
You'll find regen and DPF issues are prevalent and unfortunately difficult to fix. Your dealer isn't the only one having a hard time trying to diagnose and fix these systems. My dealer has been trying to figure out what is going on with mine for nearly the last 200 hrs. The current thought is a stuck thermostat keeping the engine from reaching full operating temp thus not allowing regens to take place. That is of course why I asked about your tractors engine temps.
I see, its a pain in the a... been like this for 200h as well, but just got worse. The error code it displays is for high exhaust temp, they just figured that out. Whats your machine?
Funny thing is, agco is all over the web bragging about their emission system and how maintenance and trouble free they are.