In the status report of my drupal website I got the message that I had to up
Database 4 byte UTF-8 support. I updated all my tables and database like it's told here: https://www.drupal.org/node/2754539. I checked and all my tables and the database have utf8mb4 encoding and utf8mb4_general_ci collation.
I get the following warning:
Enabled, but database tables need conversion Please convert all database tables to utf8mb4 prior to enabling it in settings.php. See the documentation on adding 4 byte UTF-8 support for more information.