Hi Afrodite,
We've done some testing and discovered a few things. It appears that something has become corrupted on your grid. We tested the following to confirm this:
1) We created a new screen with the contacts table, built a grid, and created the Excel button -- it exported without issue.
2) We copied the original CRM_CustomerList screen, added an Export to Excel button, and again it worked without issue
3) Finally, we added a new grid to the custom screen you're currently having issue with, and redirected the Export to Excel button to that, and it worked
This confirms that the issue lies in your grid. I would recommend deleting the grid and re-creating it, and then re-trying the button.
Naeem