The Event ID 24 shows that the error is generated in the 'MS Exchange Web Services'. This encountered when the certificate gets expired within an Organization. However, Event 24 would not lead to any service outage, since there must have a renewed certificate in that place. This very blog is presented on how to remove this expired certificate in Exchange Server 2010.
In the figure shown above, you can see the expired certificate. For viewing more information, go to the Organization Configuration, then Manage Federation >> Manage Federation Certificate where you can view the certificate with the expired date on which it got expired.
In the figure above, you can see the previous certificate having date matches with the one in Event Id 24. When you remove this certificate, you will get an error: "Active Certificates in use by Federation cannot be removed"
This error window shows that "Active Certificates in use by Federation cannot be removed" while removing it using Exchange Management Console.
Here, in the fig above, you can see error is also occurring when it is trying to remove by the Powershell command.
Now, for removing the expired certificate, you need to follow the enlisted steps:
In this figure, select the expired certificate & click open for checking the Thumbprint. After confirming, then its safe to delete that expired certificate as shown in the figure below.
After deleting the expired certificate, refresh the EMC from server configuration & then wait for sometime. Now, you can see the expired certificate has been deleted from there. But still, to be sure, navigate to the Organization Configuration again. There go to the Manage Federation & select the Show Distributed State option as described in the figure below.
In this figure, you can see the distribution state which shows the 'Distribution Status' i.e., how many certificates are installed, how many are uninstalled & even unreachable.
Imortant: It might happen that it takes some time to show the distribution state, so be calm & give some moment to let it load properly.