Clients unable to open Office Web Apps from SharePoint where ESP is enabled.
It's possible ESP is blocking a connection sourcing from the Office Web Apps Server, where it needs to initiate a verification connection to your SharePoint server using a GET request something similar to this URL
"https://sharepoint.kemptest.com/connect/IE/compliance/commercial/_vti_bin/wopi.ashx/files/c075547437214edeb69859c15a9c." This connection shouldn't be authenticated.
Add string */_vti_bin/wopi.ashx/* to your ESP "Pre-Authorization Excluded Directories"
Navigate to your Virtual Service > Modify > ESP > Pre-Authorization Excluded Directories, and add