I can't add or remove validators when using IBFT

[teipub]

System information
Geth version: 1.8.12-stable-a4580fc2
OS & Version: Ubuntu 16.04.5 LTS
Using 4 Node by IBFT

Expected behaviour
I want to change validators and network block generation does not stop

Actual behaviour
Mining stopped because the hard disk was full.
So I removed the node and replaced it with another node, but istanbul.propose doesn't seem to work properly.
It applies to istanbul.candidates , but nothing happens to istanbul.getValidators .
0x8077a2c9c4d01690f128fa147b7d321bf2a46181 is the failed node.

> istanbul
{
  candidates: {
    0x11aa77afb62d37448e8252ab91828816e992eba2: true,
    0x6d2bfd73a441fcb67078c3510365599efd1078cd: true,
    0x8077a2c9c4d01690f128fa147b7d321bf2a46181: false,
    0x96474de4231b09c51c793a7e21ff4116b76f18ab: true,
    0xf5b35b2d14e49ecd3323a1f0b6cc17452e5027b2: true
  },
  discard: function(),
  getCandidates: function(callback),
  getSnapshot: function(),
  getSnapshotAtHash: function(),
  getValidators: function(),
  getValidatorsAtHash: function(),
  propose: function()
}
> istanbul.getValidators("latest")
["0x8077a2c9c4d01690f128fa147b7d321bf2a46181"]

Backtrace

DEBUG[01-26|09:26:51.595] Recalculated downloader QoS values       rtt=20s confidence=0.667 ttl=1m0s
TRACE[01-26|09:26:53.755] Catch up round                           address=0x96474de4231b09C51c793a7e21Ff4116B76F18Ab old_round=1  old_seq=25841193 old_proposer=0x8077A2C9C4d01690f128fa147b7d321bF2a46181 new_round=2 new_seq=25841193 new_proposer="&{validators:[0xc420399da0] policy:0 proposer:0xc420399da0 validatorMu:{w:{state:0 sema:0} writerSem:0 readerSem:0 readerCount:0 readerWait:0} selector:0xb3b950}"
ERROR[01-26|09:26:53.755] Failed to decode message from payload    address=0x96474de4231b09C51c793a7e21Ff4116B76F18Ab err="unauthorized address"
TRACE[01-26|09:26:55.856] Accepted connection                      addr=3.34.72.52:34158
TRACE[01-26|09:26:55.858] Failed RLPx handshake                    addr=3.34.72.52:34158     conn=inbound    err="ecies: invalid message"
TRACE[01-26|09:26:55.858] Setting up connection failed             id=0000000000000000 err="ecies: invalid message"
TRACE[01-26|09:26:57.296] Accepted connection                      addr=15.164.9.216:52842
TRACE[01-26|09:26:57.297] Failed RLPx handshake                    addr=15.164.9.216:52842   conn=inbound    err="ecies: invalid message"
TRACE[01-26|09:26:57.297] Setting up connection failed             id=0000000000000000 err="ecies: invalid message"
TRACE[01-26|09:27:01.755] Dial task done                           task="wait for dial hist expire (29.996655866s)"

[antonydenyer]

When you propose a new validator it needs to be done from existing validators. Check what you have in istanbul.getValidators() from an existing validator (i.e node 0) and confirm that you called istanbul.propose("0xNewValidatorAddress") from more than half of those validators ie 3 of them in your case