ORA error when admin wants to view the uploaded files from students (Assignments - Advanced uploading of files)

Some of our students have uploaded files, but neither teachers nor admins can view these files...
(/mod/assignment/submissions.php?id=x)

The following ORA error (see screenshot) is displayed, if developer-messages is switched on.

ORA-00918: Spalte nicht eindeutig definiert (column ambiguously defined)

SELECT u.id, u.id, u.firstname, u.lastname, u.picture,
s.id AS submissionid, s.grade, s.submissioncomment,
s.timemodified, s.timemarked FROM m_user u LEFT JOIN m_assignment_submissions s ON u.id = s.userid
AND s.assignment = 1 WHERE u.id IN (4,5) ORDER BY lastname ASC

• line 677 of lib/dmllib.php: call to debugging()
• line 918 of lib/dmllib.php: call to get_recordset_sql()
• line 1116 of mod/assignment/lib.php: call to get_records_sql()
• line 560 of mod/assignment/lib.php: call to assignment_base->display_submissions()
• line 43 of mod/assignment/submissions.php: call to assignment_base->submissions()