Exchange HTTP error 400 and event ID 15021 after a reboot

After a reboot, my Exchange servers started returning HTTP ERROR 400. The cause was not obvious at first. The event log had a series of events with ID 15021:

An error occurred while using SSL configuration for endpoint 0.0.0.0:444.
The error status code is contained within the returned data.

Finding the cause

The message is not very helpful, but it mentions SSL and the binding on port 444. Exchange back-end services listen on ports 81 and 444, so I checked whether the service was healthy by connecting to port 444 locally on one of the servers. That failed with HTTP ERROR 503.

In the IIS console, the reason became clear: the back-end site had no certificate assigned.

The fix

Assign the certificate to the back-end binding again in IIS:

1. Open Internet Information Services (IIS) Manager.
2. Select the server and expand Sites.
3. Right-click Exchange Back End and choose Edit Bindings….
4. Select the https binding on port 444 and click Edit….
5. Under SSL certificate, the list probably shows Not selected, or the wrong certificate.
6. Click Select… and pick the correct certificate. View… lets you check that it is the right one.
7. Run iisreset.

Exchange then answers normally again.

Related

• Load balancing Exchange with HAProxy