All of these sentences are wrong. Each one contains a grammatical error. The author attempts to use "but" with a valid meaning but his attempt fails. I agree with Andrew's interpretation of what the author was TRYING to say. Here is why I believe the author failed.
"But" can be used both as a conjunction and also as a preposition. In both cases it serves the purpose of creating exceptions. Here are examples of both:
CONJUNCTION:
"The cookies look good but they are too sweet."
"Nobody knew him but didn't love him." (a good way to say what the author was trying to say)
PREPOSITION:
"All the cookies look good, but one."
"I have no life but this."
"All the guests but Bill were polite"
"Nobody who knew him felt anything but love" (another good way to say what the author was trying to say)
When you use "but" as a preposition, you need to provide the preposition an object, and that object needs to be a noun (such as "one", "this", "Bill", and "love" in the above examples.)
In the sentence "Nobody knew him but loved him", "but" tries to simultaneously be a conjunction and a preposition, and thus fails at both. If you interpret it as a conjunction then it doesn't have the meaning you want it to have. If you interpret it as a preposition, it lacks an object so the listener thinks "loved" is a verb and starts to wonder: "but WHO loved him? Nobody?"
I think most people will probably catch the meaning anyway, but it has the potential to confuse.