The problem is not with Web Viewer because it is producing an XML file successfully to completion. An .XML file is a valid EXCEL extension. However, after workstations are upgraded with Windows 10 or Office 365 Pro Plus the default app for .xml is MS Office XML Handler which has the problem. We have worked with Microsoft Support on this for a number of months, but there was no resolution.
For Windows 10, you will need to go to Programs_>Default programs->Set Associations.
Apply the latest Web Viewer Maintenance build to resolve a different problem with export and Windows 10 with the export wizard creating extra columns.
See How to locate the latest build (maintenance) for Web Viewer 12.1?
Note - this link shows the download for Windows server, but all links for all platforms are the same so this can be used on any platform