Again, you're probably bumping up against some timeout somewhere along the lines, or perhaps even running the server/workstation out of disk space as it tries to create a really big excel document.

If all they are doing is exporting the data so they can sort/pivot/aggregate it, why not do that in the report for them? Why have the extra step of exporting all of that data just to have most of it tossed aside by aggregation?

-Luke.