Redirect OWA between 2 CAS servers

Hi there,

We are running Ex2010 SP1 with DAG.  We also have 2 AD sites which has MBX, CAS/HUB on each site. Both CAS servers configured external URL like mail.company.com/owa. Internal URL: cas1.company.local/owa and cas2.company.local/owa

User1 has mailbox on CAS1 (site 1), user2 has mailbox on CAS2 (site 2)

User 1 can OWA without any issue

User 2 gets this error when OWA: 

"A Server Configuration change is temporarily preventing access to your account.  Please close all Internet Explorer windows and try again in a few minutes.  If the problem continues, contact your helpdesk"

If I move user2 mailbox to CAS1 then no error anymore. MS Outlook is working fine in both cases.

Please help.

Thanks,