Whats This Type Of Error Called

I have this stamp from austria with this white line vertically across the whole stamp.
What caused it. Do you have any ideas.
compare it to the stamp.

best regards mrmagic

sry for the blurring on the images, they're just fotos not scanned images of the stamp.

Perhaps a scrap strip of paper got trapped on top pf the actual stamp paper for a moment in the printing press?

Welcome by the way!

If the cause was a paper fold, the width of the stamp would be slighter wider than the norm. I think a narrow strip of paper covered that area when it was printed

Thanks for all your answer guys.
i#7743; 100 percent sure that the cause isnt due to a fold.
The surface is as smooth on the white area as the rest of the stamp, it must have occureed during the printing somehow.
Itś a nice oddity to have .
Iḿ with you Puzzler on the theory that something most have been on the top during printing.
Thanks by the way iḿ glad to be here with you guys.

Best regards mrmagic

Very attractive printing flaw, thanks for sharing.