Error downloading reports in Excel Format

Hi

My colleagues and I are having issues downloading reports in Analytics - we select the relevant options (download > excel), it comes up with the "Your report is being generated in the following format: Excel" and then shortly turns into the following error message:  Failed to create XLS report. An administrator has been notified.

It's been like this for weeks. Workspace reports download perfectly fine and the scheduled projects come out on time but the main analytics reports are all affected by the above.

Thanks

Kirsten