I had this happen even when mining to my local pool when I had it online ...
If you are USA use dmd.miningfield.com, it is closer (Germany) than dmdpool.digsys.bg (Bulgaria).
If there is enough demand I may re-open my pool here in Utah, USA. This will require a pool fee for me to get static IP address ...
I have three mining rigs running constantly and I only get this message on one of them. I have tried almost everything to fix the problem and have had no success. Sometimes will get get this message over 100 times a day. Other times just a few times. But the key point is I only see it on 1 mining rig and all 3 are basically the same configuration.