That looks like the character encoding of the database, table or even column may be different to what is expected (or possibly between the old server and the new one if you’re moving the backup from an old machine to a new one), so you’ve got multi-byte characters that MySQL/MariaDB can’t make sense of in to convert to utf8. These sort of things can take some hunting down.
My first suggestion would probably be to check what the /etc/my.cnf file(s) say on both the old and new server, and make them the same to start with (don’t change the old server obviously - make the new one match it!).
This question has come up before and @knation pointed to a stackoverflow thread about it (as its really a database issue, rather than RT itself).