Hi Geoffrey.

Is this when you click on export content in the "admin menu" or on the left in the "export content menu"?

I'am not sure but I think it has been successfully used on Mambo 4.5.2 before so it may be a corrupted file.

You could try uninstalling and reinstalling Export content.
and check that there have been no core hacks on the mambo install.

Also check that the MySQL Database Prefix is "mos_" under global configurations.