A family of Microsoft word processing software products for creating web, email, and print documents.
I had the same problem on PC but I finally fixed it! It wasn't a border, I tried all the basic and advanced tricks I knew and then went online. Nothing.
Then I went to the header and clicked "remove header." And it was gone! Despite the fact that it didn't in the slightest look like it could be in the header. The header margin was narrow and the line was waaaay below it. But this worked for me--I hope it helps you!