it never fails, it never fails… or rather it always seems to fail:  exchange service packs or rollups.  I’ve had to fix quote a few at work and in my home lab.  one thing that I’ve come across multiple times for exchange 2010 is to run a powershell script that automagically fixes the issue.  this post has it right:

Typical, i spend ages looking about with no joy but as soon as i post i find a solution!
For anyone else with the same issue;
"After you install update rollup 1,2 or 3 on an Exchange 2010 Client Access Server you often get a blank OWA page when browsing to the OWA page.
After installing the rollup updates you will receive something like the following URL;https://mail.msexchangeblog.nl/owa/auth/logon.aspx?url=https://mail.msexchangeblog.nl/owa/&reason=0 .
To fix this issue you must start updatecas.ps1 in the Exchange Management Shell. You can find the script in C:\Program Files\Microsoft\Exchange Server\V14\Bin . The script updatecas.ps1 will handle the OWA and ECP updates. The updatecas script comes with the update rollup."

That’s it.