After you click on export to excel. wait for some time for the pop up to come up.... Even after some time if it doesn't come up and throw a timeout error "Missing sEntry parameter", then you can try by increasing the time limit. But I won't recommend this since, this is happening due to huge volume of data.
Below thread might help you
Error when exporting WebI to PDF
If you export it using rich client, you can clearly see that the file size is too large.. It was 45 MB in my case. So I requested my user to put some filters and then do an export to excel.
