There are some things you can go checking out.
One, check your "my.ini" file in the MySQL installation folder.
Check the entries:
[mysql]
default-character-set=utf8
[client]
default-character-set=utf8
if you change them, don't forget to backup first, and restart the service later.
Another attempt is that you remove the
characterEncoding and
useUnicode parameters from your connection string, so that MySQL uses the default ones, and you have a better clue where the error comes from.