I login as a teacher, editing my lesson.
At the administrator block, I clicked the GRADE link and I got....

[Microsoft][ODBC SQL Server Driver][SQL Server]Error converting data type nvarchar to bigint.

SELECT COUNT(DISTINCT u.id) FROM mdl_grade_grades g RIGHT OUTER JOIN mdl_user u ON (u.id = g.userid AND g.itemid = lastname) LEFT JOIN mdl_role_assignments ra ON u.id = ra.userid WHERE ra.roleid in (5) AND ra.contextid IN (24,20,1)
line 686 of lib\dmllib.php: call to debugging()
line 379 of lib\dmllib.php: call to get_recordset_sql()
line 361 of grade\report\grader\lib.php: call to count_records_sql()
line 140 of grade\report\grader\index.php: call to grade_report_grader->get_numusers()