Sorry mate, that is a tough one to answer without knowing details. However, this forum entry might help you if you haven't had a look yet: moodle.org/mod/forum/discuss.php?d=374827 Changing 'dbcollation' in the moodle config file (config.php) from 'utf8mb4_unicode_ci' to 'utf8_general_ci' seems to work for most people. Sorry I am not much help here.
