Hilchos Lashon Hara chapter 7, section 7

Now we will begin to explain the rules of accepting lashon hara from a person who is as believable as two witnesses, from someone who mentions it casually without meaning to report (mesi'ach lefi tumo), or when there are clear signs that the story is true.

Accepting lashon hara is forbidden even if he heard it from a person he trusts as much as two witnesses.

In chapter 4, section 5, we wrote that one may quietly tell one's rabbi or close confidant, if he trusts him like two witnesses. That was about a case where, as long as the person has not done teshuva (repented), it is allowed to speak about his wrongdoing. For example, he did on purpose a sin that every Jew knows is forbidden, and there is no way left to judge him favorably.

But here we are talking about something that can be judged favorably. For example, maybe he did not know it was forbidden, or maybe he sinned by mistake. In such a case, it does not help that the speaker is as believable as two witnesses. Because even if the story is true, the Torah requires us to judge him favorably.