Schematics odd error message

Hi!

I'm experienceing an odd error message in Creo Schematics 3 during sheet export. There are a few Block diagram sheets in the design, and when I try to export all, I get the following error message:

the point is: "Field section not defined for type UNSET"

I've managed to narrow it down to two sheets, all the rest is exportable, but there is this two I've found which gives this error. Nothing special on theese except maybe the highways (I've never used Highways before on multiple sheets with the extend command, so this is the only thing I'm suspicious about, although I don't see why it would be a problem, the design works fine by the way).

Does anyone know why is this happening, or experienced the same?

Thanks for your help!

Regards,

Laszlo