When filtering from the Item analysis screen in the hotpot module (mod/hotpot/report.php?id=x&mode=analysis), we get the following error (with the Debugging developer activated):

Debug info: ORA-00918: column ambiguously defined
SELECT *
FROM (SELECT
ha.*, (ha.timemodified - ha.timestart) AS duration, ROUND(gg.rawgrade, 0) AS grade, u.id AS userid, u.firstname, u.lastname, u.picture, u.imagealt, u.email
FROM m2hotpot_attempts ha, m2grade_items gi, m2grade_grades gg, m2user u
WHERE ha.hotpotid=:o_hotpotid AND ha.userid=gg.userid AND gg.itemid=gi.id AND gi.courseid=:o_courseid AND gi.itemtype=:o_itemtype AND gi.itemmodule=:o_itemmodule AND gi.iteminstance=:o_iteminstance AND ha.userid=u.id
ORDER BY u.firstname, u.lastname, ha.attempt ASC)
WHERE rownum <= :o_oracle_num_rows
[array (
'o_hotpotid' => '1',
'o_courseid' => '124',
'o_itemtype' => 'mod',
'o_itemmodule' => 'hotpot',
'o_iteminstance' => '1',
'o_oracle_num_rows' => 30,
)]
Error code: dmlreadexception
Stack trace:
line 407 of /lib/dml/moodle_database.php: dml_read_exception thrown
line 274 of /lib/dml/oci_native_moodle_database.php: call to moodle_database->query_end()
line 1101 of /lib/dml/oci_native_moodle_database.php: call to oci_native_moodle_database->query_end()
line 1362 of /lib/tablelib.php: call to oci_native_moodle_database->get_records_sql()
line 152 of /mod/hotpot/report/renderer.php: call to table_sql->query_db()
line 88 of /mod/hotpot/report/renderer.php: call to mod_hotpot_report_renderer->reportcontent()
line 93 of /mod/hotpot/report.php: call to mod_hotpot_report_renderer->render_report()

We've Moodle 2.3.1 and Oracle.

To fix this problem we've edited the mod/hotpot/lib.php file and, in the function hotpot_get_recent_mod_activity, we've changed the following line (645):
$select = 'ha.*, '.$duration.', u.firstname, u.lastname, u.picture, u.imagealt, u.email';

by this other:
$select = 'ha.id, ha.hotpotid, ha.starttime,ha.endtime, ha.score, ha.penalties, ha.attempt, ha.timestart, ha.timefinish, ha.status, ha.clickreportid, ha.timemodified, '.$duration.', u.firstname, u.lastname, u.picture, u.imagealt, u.email';

because the problem is the ha.userid column which also is defined in the $filter variable as u.userid.