WooCommerce Invoice Error

Hey hello dear Woo-Team,

I have a question on the pdf invoice function.

An order is made and the customer receives the pdf invoice attached to a mail. That works like a charm.
But when I try to open the invoice in our WooCommerce order overview page http://puu.sh/pHQO2/953d401af6.png (Woocommerce–> Orders) the invoice cannot be created.
I click on the invoice number and get the following error: http://puu.sh/pHQuK/583252c65a.png in Firefox.
If I do the same in the Internet Explorer: I can download a file but it is damaged beyond repair.

Can someone help me figure out what the problem might be?
That would be magic.
Thanks a lot,
Cheers,
Claudia

https://ww.wp.xz.cn/plugins/woocommerce/