I cannot even include the url in a normal iframe.

If you see https://www.tinywebgallery.com/blog/advanced-iframe/free-iframe-checker

Enter your url there. there you will see they have: 

x-frame-options: ALLOWALL

Which is a not supported setting! 

See: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Frame-Options

So the problem is that they have set a invalid x-frame-options header! 

So please contact  them that they fix their page!

Best regards, Michael

Well when I use the iframe code they provide into my WP website this is what I see, so the iframe code dose work for me and I've tested it on other browsers and made sure I'm not logged into their website.

But I will ask them to see what they can do

I do not know why but now on the iframe checker page I see the content. 

When I click on a link I see

Refused to display 'https://tennisfriends.challonge.com/' in a frame because it set 'X-Frame-Options' to 'sameorigin'.

Which is blocking then the link. 

on https://tennisfriends.challonge.com/brackettest01/predictions/module they have a different X-Frame-Options then on the other pages.

So you need to contact them to fix this properly! 

Best regards, Michael

also a different link to https://challonge.com/de/users/ajpalmer2508 has the same problem:

Refused to display 'https://challonge.com/' in a frame because it set 'X-Frame-Options' to 'sameorigin'.

So they need to fix this for this domain as well.

Best regards, Michael

Well, I'm glad it's not me at least 😀 

I'll get onto them and see if they can fix it.

Thanks for looking into this anyway it's put my mind at rest that there is nothing on my end I can do.