As part of the upgrade, the par of data of the *_settings tables is being serialized as JSON, rather than with PHP serialized(), as it was before. During this process, the migration script tries to unserialized the data but fails. At least this happens with settings for navigation menus and side blocks (I see the error when the correspondent script is executed).
This means that the data there is corrupted. The message isn’t specific but according to the screenshot, I’d assume an encoding issue. First of all, I’ll check the forum for similar issues, e.g.: OJS 3 Special characters no longer show since upgrade from OJS2 and Problems displaying UTF-8 encoded latin characters