The stubborn urge to check the accuracy of irrelevancies rather than to reflect on relevancy at the risk of error is one of the most widespread symptoms of a regressive consciousness.For all the awkwardness of the translation, part of what Adorno is criticising here, I think, is the less than progressive practices of contemporary academic philosophy; the way it all too easily loses sight of the point of it its own existence; the way it often fails to adopt a broad enough perspective on its own activity; the way it very quickly gets tangled in minor squabbles: "the accuracy of irrelevancies".
For Adorno, what we need is to "reflect on relevancy" itself. In other words: to think about why we're thinking in the first place. This is a simple-sounding thought, to be sure. One that is easily passed over. But let's not be so hasty, shall we?