Please try the following:

1) Make sure that the "Footer Comments" and "Report Title" are only 1 line.
At the present time multi-line footers or report titles (created by pressing 
ctrl-enter) are not supported.

2) Make sure the Start and End dates are in the same format as in your
Crontrol Panel's "Regional Settings" section.  To test you may want to
try running the report with no start or end dates.

3) If there are a lot of activities/students it is possible to make 
selection criteria which generates formulas which exceed the report 
writer's capabilities.  Try running reports without making any activity/
activity type/student selections.