Please check if you maybe have proxy settings involved.
In the past we saw problems arising from this. Network guys may have a proxy set up and standard ubuntu repos might be whitelisted - but this repo not.
If you would enter “lancelot.netknights.it/community/bionic/devel/” in your browser, you could get a 404, since the browser might decide to redirect you to “https://lancelot.netknights.it/community/bionic/devel/”. And in facht this URL does not exist!
Your browser might not redirect you, and you would get a 403, since in fact the directory “/community/bionic/devel/” is currently configured to be not directly browsable.
@asmird Hope this helps you to tract down your issue. It then would be great, if you could share, what your problem actually was.