Ejabberd TLS negotiation failed

I've been trying for two days to get a simple Ejabberd setup working on a FreeBSD system, nothing too complicated, but couldn't get past a "TLS negotiation failed" error in Conversations. I haven't even gotten to the part of worrying about STUN/TURN for calls or stuff like that, all I'm trying to do is to just get the program to work. All I did was take the default config file, add an administrator account, and add my Cerbot certificates.

Here is the jabber.yml configuration file (I took the liberty of replacing my domain name with MY_HOST and my IP addresses with MY_IPV*):

###
###              ejabberd configuration file
###
### The parameters used in this configuration file are explained at
###
###       https://docs.ejabberd.im/admin/configuration
###
### The configuration file is written in YAML.
### *******************************************************
### *******           !!! WARNING !!!               *******
### *******     YAML IS INDENTATION SENSITIVE       *******
### ******* MAKE SURE YOU INDENT SECTIONS CORRECTLY *******
### *******************************************************
### Refer to http://en.wikipedia.org/wiki/YAML for the brief description.
###

hosts:
  - "MY_HOSTNAME"

loglevel: info

## If you already have certificates, list them here
certfiles:
  - /usr/local/etc/letsencrypt/live/MY_HOSTNAME/fullchain.pem
  - /usr/local/etc/letsencrypt/live/MY_HOSTNAME/privkey.pem

listen:
  -
    port: 5222
    module: ejabberd_c2s
    protocol_options:
      - "no_sslv2"
      - "no_sslv3"
      - "no_tlsv1"
      - "no_tlsv1_1"
    ciphers: "ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256"
    shaper: c2s_shaper
    access: c2s
    starttls: true
    starttls_required: true
    tls_compression: false
    dhfile: "/etc/ssl/ejabberd/key.pem"
  -
    port: 5269
    ip: "::"
    module: ejabberd_s2s_in
    max_stanza_size: 524288
  -
    port: 5443
    ip: "::"
    module: ejabberd_http
    tls: true
    request_handlers:
      /admin: ejabberd_web_admin
      /api: mod_http_api
      /bosh: mod_bosh
      /captcha: ejabberd_captcha
      /upload: mod_http_upload
      /ws: ejabberd_http_ws
#  -
#    port: 5280
#    ip: "::"
#    module: ejabberd_http
#    request_handlers:
#      /admin: ejabberd_web_admin
#      /.well-known/acme-challenge: ejabberd_acme
  -
    port: 3478
    ip: "::"
    transport: udp
    module: ejabberd_stun
    use_turn: true
    ## The server's public IPv4 address:
    turn_ipv4_address: "MY_IPV4"
    ## The server's public IPv6 address:
    turn_ipv6_address: "MY_IPV6"
  -
    port: 1883
    ip: "::"
    module: mod_mqtt
    backlog: 1000

s2s_use_starttls: optional

acl:
  local:
    user_regexp: ""
  loopback:
    ip:
      - 127.0.0.0/8
      - ::1/128
  admin:
    user:
      - "ADMIN_USER@HOSTNAME"

access_rules:
  local:
    allow: local
  c2s:
    deny: blocked
    allow: all
  announce:
    allow: admin
  configure:
    allow: admin
  muc_create:
    allow: local
  pubsub_createnode:
    allow: local
  trusted_network:
    allow: loopback

api_permissions:
  "console commands":
    from:
      - ejabberd_ctl
    who: all
    what: "*"
  "admin access":
    who:
      access:
        allow:
          - acl: loopback
          - acl: admin
      oauth:
        scope: "ejabberd:admin"
        access:
          allow:
            - acl: loopback
            - acl: admin
    what:
      - "*"
      - "!stop"
      - "!start"
  "public commands":
    who:
      ip: 127.0.0.1/8
    what:
      - status
      - connected_users_number

shaper:
  normal:
    rate: 3000
    burst_size: 20000
  fast: 100000

shaper_rules:
  max_user_sessions: 10
  max_user_offline_messages:
    5000: admin
    100: all
  c2s_shaper:
    none: admin
    normal: all
  s2s_shaper: fast

modules:
  mod_adhoc: {}
  mod_admin_extra: {}
  mod_announce:
    access: announce
  mod_avatar: {}
  mod_blocking: {}
  mod_bosh: {}
  mod_caps: {}
  mod_carboncopy: {}
  mod_client_state: {}
  mod_configure: {}
  mod_disco: {}
  mod_fail2ban: {}
  mod_http_api: {}
  mod_http_upload:
    put_url: https://@HOST@:5443/upload
    custom_headers:
      "Access-Control-Allow-Origin": "https://@HOST@"
      "Access-Control-Allow-Methods": "GET,HEAD,PUT,OPTIONS"
      "Access-Control-Allow-Headers": "Content-Type"
  mod_last: {}
  mod_mam:
    ## Mnesia is limited to 2GB, better to use an SQL backend
    ## For small servers SQLite is a good fit and is very easy
    ## to configure. Uncomment this when you have SQL configured:
    ## db_type: sql
    assume_mam_usage: true
    default: always
  mod_mqtt: {}
  mod_muc:
    access:
      - allow
    access_admin:
      - allow: admin
    access_create: muc_create
    access_persistent: muc_create
    access_mam:
      - allow
    default_room_options:
      mam: true
  mod_muc_admin: {}
  mod_offline:
    access_max_user_messages: max_user_offline_messages
  mod_ping: {}
  mod_privacy: {}
  mod_private: {}
  mod_proxy65:
    access: local
    max_connections: 5
  mod_pubsub:
    access_createnode: pubsub_createnode
    plugins:
      - flat
      - pep
    force_node_config:
      ## Avoid buggy clients to make their bookmarks public
      storage:bookmarks:
        access_model: whitelist
  mod_push: {}
  mod_push_keepalive: {}
  mod_register:
    ## Only accept registration requests from the "trusted"
    ## network (see access_rules section above).
    ## Think twice before enabling registration from any
    ## address. See the Jabber SPAM Manifesto for details:
    ## https://github.com/ge0rg/jabber-spam-fighting-manifesto
    ip_access: trusted_network
  mod_roster:
    versioning: true
  mod_s2s_dialback: {}
  mod_shared_roster: {}
  mod_stream_mgmt:
    resend_on_timeout: if_offline
  mod_stun_disco: {}
  mod_vcard: {}
  mod_vcard_xupdate: {}
  mod_version:
    show_os: false

### Local Variables:
### mode: yaml
### End:
### vim: set filetype=yaml tabstop=8

The server starts successfully with nothing suspicious in the log (it did complain earlier about ACME not working properly, but as I don't have a proper web server and don't care about automatic certificates or web interface just yet, I've disabled it). And yet Conversations keeps complaining about a TLS error. What should I do?


In general it's a good idea to mention what version of the program you are using (it could be a bug in a specific version that is already solved). And, if it fails with a specific client (remember to mention its version too), then try also with other client to confirm the problem (for example, a recent version of Gajim or Psi clients).

In your particular case, I think the problem is easier to solve. Searching "ejabberd certificates", all tutorials mention that ejabberd wants the private-key.pem, certificate and the fullchain.pem files merged into a single one.

See https://raymii.org/s/tutorials/Ejabberd_SSL_Certificate.html#toc_2 and https://www.mimar.rs/blog/how-to-combine-pem-keys-and-certificates-for-ejabberd-usage