if you want, send me a sql-backup via mail from the working version (WB 2.8.3 ...) and i try it with a converter, but it's difficult, because, it's not possible to check everything, if you dont talk this language
other variant.... insert the content again and everything is good
maybe, it's enough, if you change the Default_charset in your config.php from the WB 2.12 from utf8_general_ci / utf8_unicode_ci to utf8mb4_unicode_ci. After that, you've to start the upgrade-script again (see link in info-window - i-Button in top-manu)