After using this documentation to set up the Export to Excel toolbar button, nothing appears to happen after the button is clicked.
When the button is clicked, the correct Ajax call goes out and a "200 OK" response is sent back with no data (Content Length = 0). No errors are seen in the trace, HostServer logs nor the SmartObjects logs.
This is caused by a missing "C:\Program Files (x86)\Open XML SDK\V2.0\lib\DocumentFormat.OpenXml.dll" on the K2 Web Server (it will likely exist on the K2 App Server).
To resolve this issue, download the following on the Web Server and perform an IIS reset: Open XML SDK 2.0 for Microsoft Office
After the IIS reset is complete, refresh the List View page and the Export to Excel button should now successfully download an Excel spreadsheet with the list view's data.