Marco,
There is something special about that forum - it no longer exists.
Java in General Intermediate and Advanced were merged into Java in General Intermediate some time back. All posts from Advanced were moved into the new forum and then the empty forum was deleted. We then renamed Intermediate to Java in General, which is what you will see in the list.
When I try going to a feed that doesn't exist in a browser, I do get prompted. Which makes sense - JForum shouldn't give any info on whether the forum doesn't exist or you merely don't have access.
In other words the solution is to delete that feed. It's not as if any new posts are going to show up in a forum that doesn't exist!
[edited to fix typo in Marco's name]