No discv4 seed nodes found

[bilogate]

System information

Gexp version: gexp version latest
OS & Version: Linux
Commit hash : (if develop)

i am using ubuntu 16 (digitalocean vps) i can sync. full chain but after that it says No discv4 seed nodes found and problem begins.
I have checked everying. Time system is ok , When i check gexp attach
net.peerCount =1

admin.peers
[{
caps: ["exp/62", "exp/63"],
id: "d33a8d4c2c38a08971ed975b750f21d54c927c0bf7415931e214465a8d01651ecffe4401e1db913f398383381413c78105656d665d83f385244ab302d6138414",
name: "Gexp/v1.7.2-stable-9c901caa/linux-amd64/go1.7.5",
network: {
localAddress: "167.xx.253.xxx:52692",
remoteAddress: "1xx.199.xxx.48:42786"
},
protocols: {
exp: {
difficulty: 11234333021058860000,
head: "0x71724511012e77de0f8f1c93434a9d5f93fc2478015b771fe3a9255c96fe29d2",
version: 63
}
}
}]

[bilogate]

i have 4 pools in production. This is first time to see this error. I have installed gexp 3 diffrent server and same error. No discv4 seed nodes found .
There is no port forwarding bla bla bla process in other ethas altcoin like pirl , ubq , etc.
Whats hell is thiss.......