Not a straight up answer, but a theory and a way to reproduce your issue. It's complicated because it involves multiple nodes, but let's see if you can follow me.
TL;DR: SickNode@X.X.X.X changed its cookie after it was connected to the cluster.
So, this is what I did… First, on a terminal I started node1 with cookie x …

$ erl -name node1 -setcookie x
(node1@my.computer)1>

Then, on another terminal I started node2 with cookie x , connected it to node1 and changed its cookie to y …

$ erl -name node2 -setcookie x
(node2@my.computer)1> net_adm:ping('node1@my.computer').
pong
(node2@my.computer)2> erlang:set_cookie(node(), 'y').
true
(node2@my.computer)3>

Then, in yet another terminal I started node3 with cookie x and pinged node1 (which resulted in a connection attempt to node2 as well, as you will see below) and then explicitely tried to connect to node2 …

$ erl -name node3 -setcookie x
(node3@my.computer)1> net_adm:ping('node1@my.computer').
pong
(node3@my.computer)2>
=WARNING REPORT==== 21-Nov-2018::15:09:07 ===
global: 'node3@my.computer' failed to connect to 'node2@my.computer'

=ERROR REPORT==== 21-Nov-2018::15:09:26 ===
** Connection attempt from disallowed node 'node2@my.computer' **
(node3@my.computer)2> net_adm:ping('node2@FERNANDO-BENAVIDES.Conyfero').
pang

What happened so far? Well, since node1 's cookie was x and node3 's cookie was x as well, they could connect. node2 was still connected to node1 but, since the cookie there was y , node3 could not connect to it.
Erlang tries to establish a fully connected mesh of nodes, so when you connect to one of them, it automatically tries to connect you to all the others.
But I wanted to be thorough so I pinged node2 from node3 and, as expected I got a pang . Also, these messages popped up on node2 :

(node2@my.computer)3>
=ERROR REPORT==== 21-Nov-2018::15:09:07 ===
** Connection attempt from disallowed node 'node3@my.computer' **

=WARNING REPORT==== 21-Nov-2018::15:09:07 ===
global: 'node2@my.computer' failed to connect to 'node3@my.computer'

And, of course, when I tried to ping node3 from node2 …

(node2@my.computer)3> net_adm:ping('node3@my.computer').
pang

But… if I try to ping node1 …

(node2@my.computer)4> net_adm:ping('node1@my.computer').
pong

That's because they're already connected and Erlang only validates the sharing of the cookie on the initial handshake.
Finally, if I try to ping nodes from node1 , I get the expected results…

(node1@my.computer)1> net_adm:ping('node2@my.computer').
pong
(node1@my.computer)2> net_adm:ping('node3@my.computer').
pong
(node1@my.computer)3>

Hope this helps.