What signaled economic weakness in the 1920s?
There was a slump in 1921
A financial crisis occurred in 1920-1921, it was very short and libertarians claim it was solved so quickly because nothing was done to prevent it from lasting(thus claiming that non-intervention in the economy was a solution).
Also the prosperity did not benefit to the whole population, indeed workers, minorities and rural areas were excluded from economic growth. Roughly 60% of the population still lived in poverty.