Slack alerts - getting error: x509: certificate is valid for, not" - Where is it going wrong?

I am testing a vanilla grafana 5.4.3 (other version have same issue) using docker instance. Our machine’s URL is in DNS and a certificate has been setup for this machine using lets encrypt. When I test the notification function to our slack Webhook, I get the error below on this specific machine. When I try the same on a different machine it works so I can confirm the webhook is valid. I checked to see whether it may be picking up a certificate by mistake but inside the container, it only has the standard set of certificates. I am unsure why I am getting the error below. Can anyone shed light on this and where I should look?

=2019-02-07T21:48:38+0000 lvl=eror msg="Failed to send slack notification" logger=alerting.notifier.slack error="Post x509: certificate is valid for, not" webhook=test

http / https proxy in the way?

Thanks @torkel for your input.

I got it to work now…

After rebooting the server (& updating Ubuntu pkgs), I re-started the Grafana docker instance but it failed as nghttpx was taking up port 3000. I stopped that service and was able to restart the Grafana docker instance. It is now working! :slight_smile: I think it may have needed the reboot to reset things after all the things I was trying. nghttpx is part of http/2 to try to fix an error I saw with triggering a slack web hook via curl. I don’t think nghttpx was the issue as Grafana already had the error prior to me trying the curl command.