I am having this problem on a mongos client that connects to my 3 config servers.
I have other mongos clients running with no problems.
Here is the log excerpt:
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Mon May 14 11:43:04 [mongosMain] connection accepted from 46.137.95.131:42679 #211
Mon May 14 11:43:04 [conn211] User Assertion: 13390:unrecognized command: replSetHeartbeat
Mon May 14 11:43:04 [conn211] AssertionException while processing op type : 2004 to : admin.$cmd :: caused by :: 13390 unrecognized command: replSetHeartbeat
Mon May 14 11:43:05 [mongosMain] connection accepted from 46.137.95.91:51782 #212
Mon May 14 11:43:05 [conn212] User Assertion: 13390:unrecognized command: writebacklisten
Mon May 14 11:43:05 [conn212] AssertionException while processing op type : 2004 to : admin.$cmd :: caused by :: 13390 unrecognized command: writebacklisten
Mon May 14 11:43:05 [conn212] SocketException: remote: 46.137.95.91:51782 error: 9001 socket exception [0] server [46.137.95.91:51782]
Mon May 14 11:43:05 [conn212] end connection 46.137.95.91:51782
Mon May 14 11:43:05 [conn210] User Assertion: 13390:unrecognized command: replSetHeartbeat
Mon May 14 11:43:05 [conn210] AssertionException while processing op type : 2004 to : admin.$cmd :: caused by :: 13390 unrecognized command: replSetHeartbeat
Mon May 14 11:43:06 [mongosMain] connection accepted from 46.137.95.91:51783 #213
Mon May 14 11:43:06 [conn213] User Assertion: 13390:unrecognized command: writebacklisten
Mon May 14 11:43:06 [conn213] AssertionException while processing op type : 2004 to : admin.$cmd :: caused by :: 13390 unrecognized command: writebacklisten
Mon May 14 11:43:06 [conn213] SocketException: remote: 46.137.95.91:51783 error: 9001 socket exception [0] server [46.137.95.91:51783]
Mon May 14 11:43:06 [conn213] end connection 46.137.95.91:51783
Mon May 14 11:43:06 [ReplicaSetMonitorWatcher] checking replica set: s01
Mon May 14 11:43:06 [ReplicaSetMonitorWatcher] ReplicaSetMonitor::_checkConnection: s01r01.mongo.shopcade.com:27018

{ setName: "s01", ismaster: true, secondary: false, hosts: [ "s01r01.mongo.shopcade.com:27018", "s01r02.mongo.shopcade.com:27018" ], arbiters: [ "s01a01.mongo.shopcade.com:27018" ], primary: "s01r01.mongo.shopcade.com:27018", me: "s01r01.mongo.shopcade.com:27018", maxBsonObjectSize: 16777216, ok: 1.0 }

Mon May 14 11:43:06 [ReplicaSetMonitorWatcher] _check : s01/s01r01.mongo.shopcade.com:27018,s01r02.mongo.shopcade.com:27018
Mon May 14 11:43:06 [ReplicaSetMonitorWatcher] ReplicaSetMonitor::_checkConnection: s01r01.mongo.shopcade.com:27018

{ setName: "s01", ismaster: true, secondary: false, hosts: [ "s01r01.mongo.shopcade.com:27018", "s01r02.mongo.shopcade.com:27018" ], arbiters: [ "s01a01.mongo.shopcade.com:27018" ], primary: "s01r01.mongo.shopcade.com:27018", me: "s01r01.mongo.shopcade.com:27018", maxBsonObjectSize: 16777216, ok: 1.0 }

Mon May 14 11:43:06 [ReplicaSetMonitorWatcher] ReplicaSetMonitor::_checkConnection: s01r01.mongo.shopcade.com:27018

{ setName: "s01", ismaster: true, secondary: false, hosts: [ "s01r01.mongo.shopcade.com:27018", "s01r02.mongo.shopcade.com:27018" ], arbiters: [ "s01a01.mongo.shopcade.com:27018" ], primary: "s01r01.mongo.shopcade.com:27018", me: "s01r01.mongo.shopcade.com:27018", maxBsonObjectSize: 16777216, ok: 1.0 }

Mon May 14 11:43:06 [ReplicaSetMonitorWatcher] ReplicaSetMonitor::_checkConnection: s01r02.mongo.shopcade.com:27018

{ setName: "s01", ismaster: false, secondary: true, hosts: [ "s01r02.mongo.shopcade.com:27018", "s01r01.mongo.shopcade.com:27018" ], arbiters: [ "s01a01.mongo.shopcade.com:27018" ], primary: "s01r01.mongo.shopcade.com:27018", me: "s01r02.mongo.shopcade.com:27018", maxBsonObjectSize: 16777216, ok: 1.0 }

Mon May 14 11:43:06 [ReplicaSetMonitorWatcher] checking replica set: s02
Mon May 14 11:43:06 [ReplicaSetMonitorWatcher] ReplicaSetMonitor::_checkConnection: s02r01.mongo.shopcade.com:27018

{ setName: "s02", ismaster: true, secondary: false, hosts: [ "s02r01.mongo.shopcade.com:27018", "s02r02.mongo.shopcade.com:27018" ], arbiters: [ "s02a01.mongo.shopcade.com:27018" ], primary: "s02r01.mongo.shopcade.com:27018", me: "s02r01.mongo.shopcade.com:27018", maxBsonObjectSize: 16777216, ok: 1.0 }

Mon May 14 11:43:06 [ReplicaSetMonitorWatcher] _check : s02/s02r01.mongo.shopcade.com:27018,s02r02.mongo.shopcade.com:27018
Mon May 14 11:43:06 [ReplicaSetMonitorWatcher] ReplicaSetMonitor::_checkConnection: s02r01.mongo.shopcade.com:27018

{ setName: "s02", ismaster: true, secondary: false, hosts: [ "s02r01.mongo.shopcade.com:27018", "s02r02.mongo.shopcade.com:27018" ], arbiters: [ "s02a01.mongo.shopcade.com:27018" ], primary: "s02r01.mongo.shopcade.com:27018", me: "s02r01.mongo.shopcade.com:27018", maxBsonObjectSize: 16777216, ok: 1.0 }

Mon May 14 11:43:06 [ReplicaSetMonitorWatcher] ReplicaSetMonitor::_checkConnection: s02r01.mongo.shopcade.com:27018

{ setName: "s02", ismaster: true, secondary: false, hosts: [ "s02r01.mongo.shopcade.com:27018", "s02r02.mongo.shopcade.com:27018" ], arbiters: [ "s02a01.mongo.shopcade.com:27018" ], primary: "s02r01.mongo.shopcade.com:27018", me: "s02r01.mongo.shopcade.com:27018", maxBsonObjectSize: 16777216, ok: 1.0 }

Mon May 14 11:43:06 [ReplicaSetMonitorWatcher] ReplicaSetMonitor::_checkConnection: s02r02.mongo.shopcade.com:27018

{ setName: "s02", ismaster: false, secondary: true, hosts: [ "s02r02.mongo.shopcade.com:27018", "s02r01.mongo.shopcade.com:27018" ], arbiters: [ "s02a01.mongo.shopcade.com:27018" ], primary: "s02r01.mongo.shopcade.com:27018", me: "s02r02.mongo.shopcade.com:27018", maxBsonObjectSize: 16777216, ok: 1.0 }

Mon May 14 11:43:06 [mongosMain] connection accepted from 46.137.95.131:42680 #214
Mon May 14 11:43:06 [conn214] User Assertion: 13390:unrecognized command: writebacklisten
Mon May 14 11:43:06 [conn214] AssertionException while processing op type : 2004 to : admin.$cmd :: caused by :: 13390 unrecognized command: writebacklisten
Mon May 14 11:43:06 [conn214] SocketException: remote: 46.137.95.131:42680 error: 9001 socket exception [0] server [46.137.95.131:42680]
Mon May 14 11:43:06 [conn214] end connection 46.137.95.131:42680
Mon May 14 11:43:06 [conn205] User Assertion: 13390:unrecognized command: replSetHeartbeat
Mon May 14 11:43:06 [conn205] AssertionException while processing op type : 2004 to : admin.$cmd :: caused by :: 13390 unrecognized command: replSetHeartbeat
Mon May 14 11:43:06 [conn211] User Assertion: 13390:unrecognized command: replSetHeartbeat
Mon May 14 11:43:06 [conn211] AssertionException while processing op type : 2004 to : admin.$cmd :: caused by :: 13390 unrecognized command: replSetHeartbeat
Mon May 14 11:43:07 [conn210] User Assertion: 13390:unrecognized command: replSetHeartbeat
Mon May 14 11:43:07 [conn210] AssertionException while processing op type : 2004 to : admin.$cmd :: caused by :: 13390 unrecognized command: replSetHeartbeat
Mon May 14 11:43:08 [mongosMain] connection accepted from 46.137.95.91:51784 #215
Mon May 14 11:43:08 [conn215] User Assertion: 13390:unrecognized command: writebacklisten
Mon May 14 11:43:08 [conn215] AssertionException while processing op type : 2004 to : admin.$cmd :: caused by :: 13390 unrecognized command: writebacklisten
Mon May 14 11:43:08 [conn215] SocketException: remote: 46.137.95.91:51784 error: 9001 socket exception [0] server [46.137.95.91:51784]
Mon May 14 11:43:08 [conn215] end connection 46.137.95.91:51784
Mon May 14 11:43:08 [conn205] User Assertion: 13390:unrecognized command: replSetHeartbeat
Mon May 14 11:43:08 [conn205] AssertionException while processing op type : 2004 to : admin.$cmd :: caused by :: 13390 unrecognized command: replSetHeartbeat
Mon May 14 11:43:08 [conn211] User Assertion: 13390:unrecognized command: replSetHeartbeat
Mon May 14 11:43:08 [conn211] AssertionException while processing op type : 2004 to : admin.$cmd :: caused by :: 13390 unrecognized command: replSetHeartbeat
Mon May 14 11:43:09 [conn210] User Assertion: 13390:unrecognized command: replSetHeartbeat
Mon May 14 11:43:09 [conn210] AssertionException while processing op type : 2004 to : admin.$cmd :: caused by :: 13390 unrecognized command: replSetHeartbeat
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