Can't send telegram notifications: "telegram: Not Found (404)"

[Teonyr]

Hey, I hope this is the correct place.

I'm running the kube-prometheus-stack in my single node k3s cluster.
I tried setting up telegram (via AlertmanagerConfig):

apiVersion: monitoring.coreos.com/v1alpha1
kind: AlertmanagerConfig
metadata:
  name: alertmanager-telegram-notifications
  labels:
    alertmanagerConfig: alertmanager-telegram-notifications
spec:
  route:
    groupBy: ["alertname","job","namespace"] # it could be other labels to be grouped by
    groupWait: 30s
    groupInterval: 5m
    repeatInterval: 1h
    receiver: "telegram"
    routes:
    - receiver: "telegram"
      matchers:
        - name: severity
          matchType: "=~"
          value: "warning|critical"
  receivers:
    - name: telegram
      telegramConfigs:
        - botToken:
            name: telegram-bot
            key: bot-token
          chatID: -100XXXXXXXXXX
          messageThreadID: 7
          apiURL: https://api.telegram.org
          parseMode: 'MarkdownV2'
          disableNotifications: false
          sendResolved: true

Using kubectl -n monitoring exec -it alertmanager-kube-prometheus-stack-alertmanager-0 -- cat /etc/alertmanager/config_out/alertmanager.env.yaml I can see, that the configuration is there:

global:
  resolve_timeout: 5m
route:
  receiver: "null"
  group_by:
  - namespace
  routes:
  - receiver: monitoring/alertmanager-telegram-notifications/telegram
    group_by:
    - alertname
    - job
    - namespace
    continue: true
    routes:
    - receiver: monitoring/alertmanager-telegram-notifications/telegram
      matchers:
      - severity=~"warning|critical"
    group_wait: 30s
    group_interval: 5m
    repeat_interval: 1h
  - receiver: "null"
    matchers:
    - alertname = "Watchdog"
  group_wait: 30s
  group_interval: 5m
  repeat_interval: 12h
inhibit_rules:
- target_matchers:
  - severity =~ warning|info
  source_matchers:
  - severity = critical
  equal:
  - namespace
  - alertname
- target_matchers:
  - severity = info
  source_matchers:
  - severity = warning
  equal:
  - namespace
  - alertname
- target_matchers:
  - severity = info
  source_matchers:
  - alertname = InfoInhibitor
  equal:
  - namespace
- target_matchers:
  - alertname = InfoInhibitor
receivers:
- name: "null"
- name: monitoring/alertmanager-telegram-notifications/telegram
  telegram_configs:
  - send_resolved: true
    api_url: https://api.telegram.org
    bot_token: <TOKEN ENCODED IN base64>
    chat_id: -100XXXXXXXXXXX
    message_thread_id: 7
    parse_mode: MarkdownV2
templates:
- /etc/alertmanager/config/*.tmpl

But notifications aren't working and I'm seeing this in the logs:

time=2025-09-02T05:18:44.619Z level=WARN source=notify.go:866 msg="Notify attempt failed, will retry later" component=dispatcher receiver=monitoring/alertmanager-telegram-notifications/telegram integration=telegram[0] aggrGroup="{}/{}/{severity=~"warning|critical"}:{alertname="KubeControllerManagerDown"}" attempts=1 err="telegram: Not Found (404)"

Since I'm stuck here for 3 days now and am out of ideas, help would be greatly appreciated. :)

Thanks

[Teonyr]

apiVersion: v1
kind: Secret
metadata:
  name: telegram-bot
  
type: Opaque
stringData:
  bot-token: "<NOT  base64 encoded TOKEN>"

Token musn't be b64 encoded. After removing the " | b64enc ", it now works.