From MySQL 4.1.15 to target MySQL 4.0.24 ("downgrade" that is)

I´ve managed to export/import my database to my new host...When I export from MySQL 4.1.15 (the old host) I simply choose compability mode MYSQL40, otherwise it won´t work in MySQL 4.0.24 (the new host). So far so good...but when I try to post a new thread/topic I get the following error message:

"File: /customers/cssforumet.se/cssforumet.se/httpd.www/post.php
Line: 288

PunBB reported: Unable to create topic

Database reported: Duplicate entry '0' for key 1 (Errno: 1062)"

When I try to answer someones thread I get the following error message:

File: /customers/cssforumet.se/cssforumet.se/httpd.www/post.php
Line: 180

PunBB reported: Unable to create post

Database reported: Duplicate entry '0' for key 1 (Errno: 1062)

The things I´ve tested to solve the problem is:
*repair database
*db management tool
*Rebuild search index via administrationpanel.
*Fresh installed copy of punBB

So is there anyone with sql knowledge? My knowledge is about "0"

Cheers!