Ubuntu – PuTTY X11 forwarding: Unsupported authorisation protocol

I have a Ubuntu 14 running as the "Server" and am trying to use PuTTY on my windows machine as the "Client"

On the Server:
ForwardX11 yes
ForwardX11Trusted yes

On the Client:
PuTTY Configuration: SSH->X11->Enable X11 Forwarding (ticked), with MIT-Magic-Cookie-1
Xming Server running

PuTTY Event Log:

2016-09-18 01:02:24 Requesting X11 forwarding
2016-09-18 01:02:24 X11 forwarding enabled

Command:

xclock

Result:

PuTTY X11 proxy: Unsupported authorisation protocol
Error: Can't open display: ubuntu3:10.0

The above command records this in the PuTTY Event Log:

2016-09-18 01:20:22 Received X11 connect request from 127.0.0.1:60167
2016-09-18 01:20:22 Opened X11 forward channel
2016-09-18 01:20:22 Forwarded X11 connection terminated

I've tried uninstalling xauth using apt-get purge xauth then reinstalling apt-get install xauth

Can anyone think of why this is not working? It was working a little while back… am not sure what changed!!?!

Command:

sudo DISPLAY=localhost:11 XAUTHORITY=/home/<username>/.Xauthority xclock

Result:

PuTTY X11 proxy: Unsupported authorisation protocol
Error: Can't open display: localhost:11

When I try the same from a another Linux box, I get this error instead of Unsupported authorisation protocol:

X11 connection rejected because of wrong authentication.
Error: Can't open display: ubuntu3:10.0

I have 2 Ubuntu systems. Named Ubuntu2 and Ubuntu3. Everything seems to work on Ubuntu2. So I started comparing the two…

xauth list

Ubuntu2

ubuntu/unix:10  MIT-MAGIC-COOKIE-1  7c27c83c47f4b323a2855c67e1eeef63
ubuntu/unix:11  MIT-MAGIC-COOKIE-1  7c1aa456987ee9face9a35f8aa72acf3
ubuntu2/unix:0  MIT-MAGIC-COOKIE-1  50073df1fac06436a5e724d93730ef64
ubuntu2/unix:11  MIT-MAGIC-COOKIE-1  5860938ccf8af0e0f4a00c37ca51feb5
ubuntu2/unix:10  MIT-MAGIC-COOKIE-1  0fa58759ec8e78b735aa3528933464c0

Ubuntu3

localhost:11  MIT-MAGIC-COOKIE-1  b6e1bf9dedae8b358133462e3d702547
ip6-localhost:11  MIT-MAGIC-COOKIE-1  b6e1bf9dedae8b358133462e3d702547
localhost:13  MIT-MAGIC-COOKIE-1  6fe13bad7d70320cf01c1d9ea76ab084
ip6-localhost:13  MIT-MAGIC-COOKIE-1  6fe13bad7d70320cf01c1d9ea76ab084
ubuntu3/unix:0  MIT-MAGIC-COOKIE-1  c4bf89ae06d1aca5f36993fe728a5d21

echo $DISPLAY

Ubuntu2

localhost:10.0

Ubuntu3

ubuntu3:10.0

which xauth

Ubuntu2

/usr/bin/xauth

Ubuntu3

/usr/bin/xauth

Something Strange

I started comparing different conf files between the two systems. And noticed ip6 was disabled on Ubuntu3 and not on Ubuntu2. So, I tried re-enabling ip6 on Ubuntu3 and no dice. So, I disabled it on Ubuntu2 and it broke X11 forwarding. I don't know why and how ip6 plays a role in this… can someone explain?