After logging into the Exchange ECP with the correct credentials you get just a blank screen with no errors messages. This will almost always happen if you remove a certificate using the certificates MMC or even sometimes if you remove one using the ECP.
Below is what you need to check to fix this problem:
Since this change is not reflected in the backend website you have to make sure that the Exchange ECP site is looking at the same certificate in both locations.
IIS Default Site SSL Certificate
IIS Back End SSL Certificate
The certificate choice in both of these locations have to match exactly then the issue is resolved.