@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.
Welcome to CiviCRM Stack Exchange ... and congratulations on finding a bug!
I've created an issue and a Pull Request (PR). It is a very simple code change so if you can apply the change in the PR, test that it works for you and then leave a comment on the PR we should be able to get this resolved.