This typically happens when a certificate is installed via the Exchange Management Shell. (EMS) When installing a cert via the EMS, you don't have the ability to specify the website the certificate is being used on, so you must tell IIS to use the right certificate via the IIS Manager.
1. Right-Click on the website that contains the OWA folder.
2. Click on Properties.
3. Click on the Directory Security tab.
4. Click Server Certificate under the Secure communications section
Enter Web Server Certificate Wizard
5. Click Next.
6. You are now presented with a few options. Click Replace
7. Select your new certificate from the list and click Next.
8. Verify the Details and click Next.
9. Click Finish to exit the wizard.
1. Using the IIS manager, navigate to the root of the Website that houses the OWA folder. The center of the window should say "Default Website Home" or whatever the name of the website is.
2. Select Bindings from under the Edit Site sub menu.
Web Site Bindings
3. Click https so that it is highlighted and then click on the Edit button on the right.
4. In the SSL Certificate section, select the new certificate from the drop-down menu.
5. Click OK to complete the Installation.
Double check your OWA site to see if the new certificate is in place!