Another 2.1 million unemployment claims were filed last week, the Labor Department reported Thursday, pushing the total past 40 million — the equivalent of one out of every four American workers — since the coronavirus pandemic grabbed hold in mid-March. The report marks the eighth week in a row that new jobless filings dipped from the peak of almost 6.9 million, but the level is still far above historic highs. The latest claims may be not only a result of fresh layoffs, but also evidence that states are working their way through a backlog. And overcounting in some places and undercounting in others makes it difficult to measure the layoffs precisely.