I’d venture to say most, if not all, make some variation of the claim “Truth is relative”, which in isolation isn’t that profound of a statement to make. But when it’s much of the foundation of your ideology it leads you to very dark places.
The proposed antidote is exactly the solution supposed by the scientific method: the intersubjective corroboration of individual observations, evidence, reasoning. However, this plainly reveals where truth, science, can fail: It is in this corroboration where atrocities in the name of some transparent, universally available truth are smuggled into society. That is why postmodern thinkers take the footing that they do.
Especially when in the post-war period it was more and more obvious how massive the contradictions were within both liberalism and marxism as they were idealized, and the world as it actually was.