Exchange 2013 OWA blank page error 400

If you encounter this problem (bad request, http 400 error) on your Exchange 2013 infrastructure.

These are the steps that you can follow to fix it.

Note:The powershell scripts are not mentioned anywhere in the official Technet documentation.
The powershell scripts are only mentioned on several blogposts regarding Microsoft Exchange 2010.

1. Login to your Exchange 2013 CAS server
2. Start the Exchange Management Shell
3. Navigate to your Exchange 2013 binaries location, for example:C:\Program Files\Microsoft\Exchange Server\V15\Bin\
4. Execute the UpdateCas.ps1 Windows PowerShell script and wait a few moments.
   This script will rebuild your OWA interface.
5. If you haven’t executed UpdateConfigFiles.ps1 , now is a good time. Execute it.
   It looks like you need to execute this Windows PowerShell script, after each cumulative update of Microsoft Exchange 2013 to keep everything working smooth.

Conclusion
After each installation of a cumulative update for Exchange 2013, remember to execute both the UpdateCas.ps1 and UpdateConfigFiles.ps1 Windows PowerShell scripts.
It will save you a lot of trouble, troubleshooting errors with OWA and ECP.