J’en ai connu means “I knew some of them/these” but without context, we cannot guess what en is referring to, it might be people, events, animals, things, whatever.

Note that normally, connu doesn’t agree with the antecedent, e.g.:

Des situations imprévues, j’en ai connu. not …, j’en ai connues.

However, once in a while agreement is done by some authors. See the oqlf.

Good question.

en is commonly called pronom neutre and serves to ease up the weight of a sentence and to avoid repetition.

• Est-ce que t’as eu des problèmes ?
• Oui j’en ai eu.

It is also defined as being partitif, meaning it serves to indicate that you’re talking about a part of a whole (quelques problèmes, de l’ensemble des problèmes) without having to specify wich.

In your example, you were talking about a specific person.

-Je le connais de nom.

Imagine however, you were asked if you ever knew people only by name.

-Connais-tu des gens que de nom ?
-Oui, j’en connais.

From all the people that could be known only by name, you know some.