Hilchos Lashon Hara chapter 6, section 10

Even though accepting lashon hara is forbidden by the Torah, Chazal said that it is allowed to be cautious about it.

This means: he may treat the story only as a possible warning, to protect himself from the person so that no harm comes from him. He should not even treat it as maybe true. So he still must do good to that person in every way the Torah commands, because the person is not worth any less in our eyes. He may only be cautious, and nothing more.

So being cautious is allowed only where harm could come to himself or to others. In any other case, it is forbidden to be cautious about lashon hara, and certainly to believe it.