1.5.0 final upgraded from 1.3.6

When updating a products description with special characters the error log shows:
PHP Warning: json_encode() [<a href='function.json-encode'>function.json-encode</a>]: Invalid UTF-8 sequence in argument in /home/domains/elektro.nl/public_html/admin/includes/init_includes/init_admin_history.php on line 34
Not sure if it's related, but I also noticed that all the database tables are latin encoded, except for admin_menus, admin_pages, admin_pages_to_profiles and admin_profiles
(admin_activity_log is encoded latin).

Other than the above error log everything seems to work fine, but I would like to keep the error log clean