Re: ERROR 1366 (HY000): Incorrect string value: '\x9B'

[Rick James]

mysql> select lastname, hex(lastname), char(lastname), char_length(lastname) from user;
-->
mysql> select lastname, hex(lastname), LENGTH(lastname), char_length(lastname) from user;
And, let's see all the rows. The interesting one is the one with the o-slash.

\x9B -- This sounds like latin1. Which says to me that
mysql> insert into user values('0000', 'a', 'a', 'a', 'ø');
was entered in latin1 instead of utf8.

Are you on Windows? Have you done
chcp 65001?

If you are on Unix, do you have a utf8-savvy terminal program?

If you can't get the 'terminal' to talk utf8, then do this:
SET NAMES latin1;
That should let the \x9B in, because it will be transcoded from latin1 to utf8 as it goes to mysqld.

Sounds like you have already gotten into this:
http://mysql.rjweb.org/doc.php/charcoll