SocketException in Mongo
Generally a connection timeout occurs from one of the following in a replica set
1) All members are not able to communicate with each other
2) A program is connecting to replica for update and it is unable to send it to primary due to overload or 1st as well
3) All relicas are not in sync and one is lagging behind too much
4) Leader election is going on but not completed due to some reason
Please check if your relica set is consistent and all nodes are working by issuing rs.status() on primary node , also as earlier suggested check primary logs for more information