Hmmm... Indeed! I have to check on that. I believe that the function opening the database connection is not a "blocking" call and when new map starts, some of the messages fired from OnMapStart event are piped to the DB before the connection is actually ready. The errors are not fatal, so your log is only left out some player connecting messages etc from the OnMapStart event... I think
I'll think of something... soon!
EDIT: It's not about not blocking calls. I'll post an update sometime soon...