Installing and upgrading help

Hi,

after upgrading Moodle from 3.1.4 to 3.1.7 and using the Scheduled task for LDAP-Snyc, I've recognized that some new users where missing, and started LDAPsysc by CLI.

 I got the following error message:

[AUTH LDAP] The users sync cron has been deprecated. Please use the scheduled task instead.
Default exception handler: Fehler beim Lesen der Datenbank Debug: Illegal mix of collations (utf8_general_ci,IMPLICIT) and (utf8_unicode_ci,IMPLICIT) for operation '='
SELECT u.*
                      FROM mdl_user u
                 LEFT JOIN mdl_tmp_extuser e ON (u.username = e.username AND u.mnethostid = e.mnethostid)
                     WHERE u.auth = ?
                           AND u.deleted = 0
                           AND e.username IS NULL
[array (
  0 => 'ldap',
)]
Error code: dmlreadexception
* line 479 of /lib/dml/moodle_database.php: dml_read_exception thrown
* line 1175 of /lib/dml/mysqli_native_moodle_database.php: call to moodle_database->query_end()
* line 765 of /auth/ldap/auth.php: call to mysqli_native_moodle_database->get_records_sql()
* line 70 of /auth/ldap/cli/sync_users.php: call to auth_plugin_ldap->sync_users()
... 

$CFG->dboptions = array(
    'dbpersist' => false,       // should persistent database connections be
                                //  used? set to 'false' for the most stable
                                //  setting, 'true' can improve performance
                                //  sometimes
    'dbsocket'  => false,       // should connection via UNIX socket be used?
                                //  if you set it to 'true' or custom path
                                //  here set dbhost to 'localhost',
                                //  (please note mysql is always using socket
                                //  if dbhost is 'localhost' - if you need
                                //  local port connection use '127.0.0.1')
    'dbport'    => '',          // the TCP port number to use when connecting
                                //  to the server. keep empty string for the
                                //  default port
    'dbcollation' => 'utf8_unicode_ci', // MySQL has partial and full UTF-8
                                // support. If you wish to use partial UTF-8
                                // (three bytes) then set this option to
                                // 'utf8_unicode_ci', otherwise this option
                                // can be removed for MySQL (by default it will
                                // use 'utf8mb4_unicode_ci'. This option should
                                // be removed for all other databases.

);

and the db.opt seems to be ok as well:

default-character-set=utf8
default-collation=utf8_unicode_ci

Where does utf8_general_ci come from? How can I correct the error?

We're using MYSQL 5.5.49 and PHP 5.6.20.

Cheers,

Monica