Not much help to you, but I have a similar problem.
Text export gives the message that it cannot find the temporary internet/Content IES/text file. It asks me if I want to create one, but then it just goes blank.
Export to Excel is even more strange, as it works well with 60 students in a course, but corrupts the Excel file with more than 60. Oddly it is not always 60 students and can go up to 73 students, depending on how many assignments are in the grades section.
Any thoughts anyone.
Mike McDermott
Moodle 1.6.1+
PHP 5.1.2