On the assignments by student view an excel file with the student submission can be exported if the sakai property timesheet.sites.allowed=all
is set. When the site has a lot of students and/or a lot of assignments, the file downloaded will be empty. This happens when the number at about over 9.200 rows - for example with 100 students and assignments, because this happens when the number of cell (styles) exceeds 64.000. The issue seems to be, that styles applied to the cells are newly created for each cell instead of being re-used. The proposed fix would be to move the creation of the cell-style out of the loops that iterate over the rows/cells, and just reuse them when the cells are created.