The links in your post don't work! Here's why.
When you insert a link in a post or PM, what you see and click on is not necessarily the actual url for the webpage linked to. For example eBay links are often several lines of text, but appear in shortened for in a post or PM ie truncated. The forum software knows what the complete url is.
What your post above actually contains is this:-
[XRL="https://www.vintageradio.net/forum/www.radiomuseum.org"]www.radiomuseum.org[/XRL]
I have changed URL to XRL to illustrate the point. Some how you've got vintageradio.net in there, so the 404 message comes from the forums.
If you want to insert a link in a post or PM it's best to copy the URL form the address bar, paste it into your post or PM and let the forum software do the rest.
www.radiomuseum.org.
Never copy a link from a post or PM.