Hi
I have a problem whith the update moodle 2.6 to 2.8. During the update moodle sytem show error

Debug info: Subquery returns more than 1 row
UPDATE mdl_tag_instance
SET component = 'core',
contextid = (SELECT ctx.id
FROM mdl_context ctx
WHERE ctx.contextlevel = ?
AND ctx.instanceid = mdl_tag_instance.itemid)
WHERE itemtype = 'user' AND contextid IS NULL
[array (
0 => 30,
)]
Error code: dmlwriteexception
Stack trace:
line 446 of /lib/dml/moodle_database.php: dml_write_exception thrown
line 974 of /lib/dml/mysqli_native_moodle_database.php: call to moodle_database->query_end()
line 3280 of /lib/db/upgrade.php: call to mysqli_native_moodle_database->execute()
line 1560 of /lib/upgradelib.php: call to xmldb_main_upgrade()
line 361 of /admin/index.php: call to upgrade_core()

Need your help
thanks