utfmb4 conversion gone wrong - Some columns in the databse have utf8mb4_general_ci others utf8mb4_unicode_ci causing and Illegal mix of collations
@mattwire helped me on chat.civicrm.org to identify the problem.
It was caused by the drupal script I used to convert drupal tables (to the wrong collation), and that script included the tables from civicrm which were listed in the drupal settings.php
I've rerun the civicrm api conversion and the drupal script properly and now everything works.