When the log 'action' => 'moodle_user_get_course_participants_by_id', is send to the database, an "insert into log failed" email is generated.
The reason why is because moodle_user_get_course_participants_by_id is 42 characters long, while the action field has only 40 characters space in my database. I'm not sure if that is the default value, because there have been issues with it being to small in the past.

So I verified my tables with moodle/admin/tool/xmldb/index.php?action=check_defaults&confirmed=yes and the log table verifies as ok (unlike a lot of others)

verifying table:
log
Veld: time OK
Veld: userid OK
Veld: ip OK
Veld: course OK
Veld: module OK
Veld: cmid OK
Veld: action OK
Veld: url OK
Veld: info OK

So either there is something wrong in the upgrade script, not altering the log action field to de correct size, or the value moodle_user_get_course_participants_by_id is too long.