After upgrading from Citrix Director 7.6 to 7.15 LTSR release I found out that exporting to Microsoft Excel was broken. See the following Error Message:
And this message in the Event Viewer:
This problem can be fixed by either running the MSI again and choosing repair. After that, it works.
What the repair does, is place an extra file in the folder: C:\inetpub\wwwroot\Director\Bin called DocumentFormat.OpenXml.dll
So, you could also find a copy of this file on a working server and copying it manually to the failing server.