I just had this happen, and it also applies to Estimates as well. The whole "content" area from the template is blank for preview, printing, or email. So I can't send any invoices to customers until this is resolved. 

Yes, all are checked in the template, and yes, I have deleted and made 3 new forms and still no luck. I go back to invoices I am waiting to get paid on that are a few weeks old, and even those the area is blank.

I also wanted to note, that I am using the "old" style invoices since I can add the custom fields and have a PO section like most customers want. Thank you