The default gradebook listing for an instructor is fine, but when a sort button is pressed, only standard page headers are shown. The various sort button URLs all include "&group=-1". When this text is appended to the URL of the standard display, an empty display results.

I believe the problem is due to near line 318 of lib.php, where $group is compared to zero instead of -1.

See attachment for a context diff of a fix.
--James