I also have the same problem on 2 of my 8 computers that are mining since day one. The other 6 are working great. It happened when the difficulty fall down to ~0.8. All 8 of them are on win10 x64.
Any help would be appreciated!
You will have to resync these affected nodes from scratch.
Looks like they got stuck!
All of my nodes and the blockexplorer are fine though, so I'm not sure what exactly caused this.
A handful of people reported this to me, but it seems to be isolated.
An investigation has been started, but for now please resync and
run as many FULL NODES as possible (open Port 51339, see that you get atleast 50 connections).
The better you are connected to the rest of the network the less you run the risk of creating (or jumping on) a fork and getting stuck there!