How to Edit Microsoft Office files from SharePoint when ESP is enabled
This Knowledge Base article applies to Clients that are unable to open Microsoft 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 the 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 the ESP "Pre-Authorization Excluded Directories"
Navigate to the Virtual Service > Modify > ESP > Pre-Authorization Excluded Directories, and add: