Instead of the graph on this URI:
/report/stats/graph.php?mode=2&course=30&time=44&report=8&userid=0

I get this error:
Error reading from database

Debug info: Unknown column 'statsreads' in 'field list'
SELECT id,roleid,timeend,statsreads+statswrites as line1 FROM mdl_stats_monthly WHERE courseid = 30 AND stattype = 'activity' AND timeend >= 1349049600 ORDER BY timeend DESC
[array (
)]
Error code: dmlreadexception
Stack trace:
line 443 of /lib/dml/moodle_database.php: dml_read_exception thrown
line 1014 of /lib/dml/mysqli_native_moodle_database.php: call to moodle_database->query_end()
line 84 of /report/stats/graph.php: call to mysqli_native_moodle_database->get_records_sql()

In other courses the graphs are ok.