Chrome browser users are unable to edit SharePoint documents in Excel. The document opens, but when the user clicks the button to edit the document in Excel, the expected pop up never appears. This is not affecting IE users, and is only affecting external users who go through an external reverse proxy.
The following header, set in the OWA SharePoint farm, was causing the problem:
Content-Security-Policy: default-src https: data: 'unsafe-inline' 'unsafe-eval'
Release: MSPSSO99000-12.8-Single Sign-On-for Business Users-MSP
Setting the value of this header to blank (no value) resolved the problem.