Slack alerts stopped arriving after we recreated the channel

That message comes from Slack, not from us, which narrows it down nicely. Slack invalidates an incoming webhook when its target channel is archived, and generating a new webhook in the Slack UI does not update the old one. Here is the fix: 1. In Slack, go to the app configuration for your incoming webhooks and confirm the webhook is bound to the new channel and is listed as active. If it still points at the archived channel, create a fresh one. 2. Copy the full URL. It should look like https://hooks.slack.com/services/T***/B***/*** with three path segments after services. A truncated copy is a very common cause. 3. In MonoDuty, edit the notification channel and paste the URL into the url key of the channel configuration. Save. 4. Fire a test incident, for example from a disposable alert trigger, and re-check the delivery records. A successful delivery is recorded as sent. One thing to be aware of while debugging: there are no automatic retries. Each notification is attempted once, so the alerts that failed during the broken period are gone rather than queued, and fixing the URL only affects future alerts.