403 Forbidden You don't have permission to access this resource?

I have the following problem.

when I try to go to the German page of W6-Wertarbeit.de I get the following message.

Forbidden
You don't have permission to access this resource.

Apache server at http://www.w6-wertarbeit.de port 443

However, I only have this error in my home network, as soon as I switch off my WLAN and try using the mobile data, the page opens. It is not the browser or my laptop, because I have the same problem on my mobile phone and tablet.

Nothing is locked in my Fritzbox and I have no restriction inside.

I have tried all devices and all browsers. As soon as I'm in my network, I do not come to this page.

I'm not an expert, but I would turn on Vpn and then see if it works.

That does not work either. It's just a website. Everything else works. It also works via my Citrix connection from the company. As soon as I'm on my network. It is no longer possible.

Hmm funny I get on the page.

I also come to the site via other networks. It just doesn't work over my private internet. It must be due to some setting on my Fritzbox. Since I can access it via the mobile network and also via my home office Citrix connection. Only when I'm on my network, it doesn't work.