I had this problem and think I have the solution - at least I was able to solve my problem.
My November updates kept failing - despite numerous downloads from the website and uploads to SV. Finally, I waited for the December cycle - same thing.
On reviewing my history, I found that I had signed up for the Seattle Avionics chart subscription. This service downloads chart updates to your thumbdrive for airport info. I had left the thumbdrive installed in the airplane. I brought it home and sure enough, it was out-of-date, and downloaded a large nuber of update changes.
When I reinstalled in the airplane - with no changes made to the Dynon data, the OUT-OF-DATE nag message no longer appeared.
So, for me - with a chart data subscription - I determined BOTH the Dynon database and the Chart database must be kept up to date to avoid the nag message.
I'm not sure if leaving the thumbdrive out still generates the nag message - in other words, once you've told SV you have a chart subscription, it remembers between uses and looks for it. I'll have to play with that part to make a determination.
So, if you have a Chart subscription and have not updated it via the Seattle Avionics downloader, that might be your problem. If you don't have a Chart subscription, then there's another problem.
Bob Bogash
RV-12
N737G