Advanced Guestbook MySQL Error

Hey, maybe anyone can help me here !

I have a major problem. As soon as I enter a second entry into my guestbook a page appears saying MySQL Error: Query Error and Error Number: 1 Can't create/write to file '/tmp/#sql7dbe_1a0843_0.MYD' (Errcode: 28)
I gave the right permissions to folders public and tmp. I can edit, delete... all messages in the admin, so reading from the database works. Does anyone know what the problem could be??

Would be awesome. thnx