Dear BrainStorm,
some of these errors you posted are the result of wrong database content, meaning that a row in a table e.g. is missing, or not needed, or contains wrong flags, etc.
E.g. you posted
2011-03-19 19:34:49 >> Loaded 10 creature reactions at game events
2011-03-19 19:34:49 ERROR:`game_event_quest` game event id (9) not exist in `game_event`
This tells you, that in the table game_event_quest a quest is listed which has a world event as condition which is not defined in your database table game_event.
Usually you can take on such error messages word by word, since they pretty much tell you what is wrong.
In many cases UDB/YTDB, or any other database provider may have a fix on their forums, and in some, you will have to browse our Wiki for database documentation and correct errors yourself.
Should you have no idea how to do that, try learning SQL, which is the language used to describe the MaNGOS databases and their content.