I upgraded a client’s CAS servers recently from Exchange 2007 Service Pack 2 Update Rollup 5 to Service Pack 3 Update Rollup 13 in preparation for upgrade to Exchange 2013. Following the restart after the Update Rollup had completed I found I could not log into OWA. Instead I got a blank page with a URL similar to the one shown below.
https://MyCAS/owa/auth/logon.aspx?url=https://MyCAS/owa&reason=0
I found several suggestions for what might be the problem on the web, but the one that worked for me was to run the UpdateOwa.ps1 Powershell script in the Bin folder under the Exchange installation path.
It was required on both CAS servers, so at least it was consistent 🙂