Hi Karel,
Check whether the three PIAs(Microsoft.Office.Interop.Excel.dll, Microsoft.Vbe.Interop.dll and Office.dll) required by the export to Excel are included in your installation/deployment. They should be placed in the same folder where all other extension dlls reside.
The other possibility is to update your installation to install also the three PIAs in the gac. This is what actually our installation does.
If you have any other questions, remarks do not hesitate to contact us.
Sincerely yours,
Support Team