One site, MB and CAS server all in one site. OWA worked with no issues.
Moved mailbox to another Site and OWA broke.
I had to change the settings on the remote location (MB Locations) for OWA:
Server configuration – > Client Access -> go to the server in question (the MB server) -> OWA (Default Web Site) -> Authentication -> change to “Integrated Windows Authentication” and on general tab, set external URL to nothing.