https://www.reuters.com/article/us-retail-trading-cash-app/squares-cash-app-says-clearing-broker-resumes-buying-of-amc-nokia-stocks-idUSKBN2A228B

Cash App's halt came on the heels of online brokerage Robinhood placing similar constraints on shares of some companies, as clearinghouse deposit requirements tied to trading volumes jumped and strained its finances. Robinhood on Monday tapped almost $3.4 billion in emergency funding from existing investors to manage the problem.