[2020] Can't connect to 9.0 server

Yes, thats the first thing to try for such issues. Make sure to backup your Storage directory though, in case you want to keep your old worlds.

Steam doesn't remove mod files, for example. And it tends to forgot to remove other stuff as well, so a clean install is always the best solution with problems on a major update.

1) Uninstall Eco
2) Backup Steam/steamapps/common/Eco/Eco_Data/Server/Storage
3) Remove the complete Eco folder mentioned in 2)
4) Reinstall the game
5) Restore backup from 2)