uft8mb4 only means that the database is able to store all kinds of UTF-8 sequences. However it seems the strings you are trying to store is not valid UTF-8. If you are using Windows the default charset is CP1252 and not every CP1252 string is automatically a valid UTF-8 string. You can try to add
-Dfile.encoding=UTF-8
to your knime.ini (at the bottom). Also if you post your workflow we could check whether we can work around such situations automatically.