This may or may not be it, but I have seen similar errors where your database character set does not align with the database settings in config.inc.php. Do you get the same message if you try to paste a greek letter or certain accented characters into your abstract? If so, this may be it.
The default database character set in mariadb for example is latin1. But for OJS it’s utf-8. So if you use the default settings for both, you’ll end up with a mismatch between your database settings and how OJS is trying to connect.
You can see instructions for checking your database here. And if you need to change your database settings you can find instructions here.