If your OWA or ECP Site are showing blank pages after Login and your Outlooks are having problems connecting to the server, then its worth to check the certificates of the „Exchange Back End“ in the IIS.
I have heard of cases were the IIS lost the information which certificate has to be used for the Back End after a server crash. I myself experienced the same after replacing an outdated self-signed certificate and forgot to change it for the Exchange Back End, too.
Right-Click on „Exchange Back End“
Click „Bindings“ (the screenshot is from a German system…)
If you see „Not selected“ than you have to choose a valid certificate and apply the changes.
Now you should be able to use the OWA / ECP site.