Elasticsearch node can't connect to cluster

Question

First of all, I want to be clear that I looked at several guides and suggested similar questions before opening this post, but none of them worked for our case.

Here is our situation:

Last week, our Elasticsearch stopped adding new records as it reached the state where the disk was almost full. We changed the config to get some time and now is working as expected. However, we want to add a new server with elasticsearch and form a cluster as we don't want to resize the disk because we don't want to lose anything.

Here is the configuration for the main server /etc/elasticsearch/elasticsearch.yml

cluster.name: my-cluster
node.name: master-node
path.data: /var/lib/elasticsearch
path.logs: /var/log/elasticsearch
network.host: 0.0.0.0
http.max_content_length: 100mb
discover.seed_hosts: ["ip_address_server_1", "ip_address_server_2"]
xpack.security.enabled: true
xpack.security.transport.ssl.enabled: true
xpack.security.http.ssl.enabled: true

As I said, we created another server with the same elasticsearch version (7.6.2)

cluster.name: my-cluster
node.name: another-node
node.data: true
node.master: false
path.data: /var/lib/elasticsearch
path.logs: /var/log/elasticsearch
network.host: ip_of_server
discover.seed_hosts: ["ip_address_server_1", "127.0.0.1"]

We tried restarting nodes at the same time, curls and everything but they can't see each other. At this point, I'm starting to think that the problem is the SSL configuration on the master, but I am not an elasticsearch expert, so I don't know what is exactly happening.

EDIT:

I took a look at the logs and here is what I found: Node 2

[ico-elastic-node-2] master not discovered yet: have discovered [{node-2}{G5SfrEv0RxaxmYf8urIFtQ}{XpfNNouCQx2HmYfw2AvoQw}{ip_server_2}{ip_server_29300}{dil}{ml.machine_memory=12558602240, xpack.installed=true, ml.max_open_jobs=20}]; discovery will continue using [ip_server_1, 127.0.0.1:9300] from hosts providers and [] from last-known cluster state; node term 0, last-accepted version 0 in term 0

And the master:

[master-node] exception caught on transport layer [Netty4TcpChannel{localAddress=/ip:9300, remoteAddress=/ip:36660}], closing connection
io.netty.handler.codec.DecoderException: javax.net.ssl.SSLHandshakeException: No available authentication scheme

Currently master is yellow. What can we try next?