Again, another odd issue I've come across this week....
While installing an Exchange 2007 CAS server at a client site (getting ready for a 2003 to 2007 migration) we encountered a case where Outlook Web Access produced the following error:
A refresh of the page resulted in a white page with no formatting and some odd looking buttons. Exchange 2007 CAS server is supposed to proxy the client requests to either a 2003 or 2007 mailbox depending on where it "lives". During the implementation of of the CAS server we tested moving a user's mailbox to a 2007 server and logged onto the CAS just fine.....no issues. It was when we attempted to authenticate a user who had a 2003 mailbox where things went sideways.
We tested navigating to the 2003 front-end server and directly at the 2003 mailbox cluster with no issues. I noticed a plug-in was installed when I visited the OWA site which lead me to ponder the possibility of it interfering with the proxy request. A call to Microsoft support confirmed this.
The 2003 mailbox server had an ISAPI filter on the IIS web site used for OWA. The plug-in is called "EASOWA" and is used by the ZANTAZ software (for email archive). We removed the ISAPI filter and it worked immediately.
Since the migration will be short, there will be no workaround for the issue and an EASOWA plug-in for 2007 is being tested as I write this.
Troubleshooting this issue was particularly difficult in that a direct connection to the 2003 FE or MB server worked fine. The 2007 CAS server also worked fine when connecting to the 2007 MB server. It was the process of proxying the link where things broke down. I was convinced it was the 2007 CAS server at the time but obviously I was wrong. Hmmm....that's twice now.