Greensill’s problems came to a head Monday after Credit Suisse’s asset-management arm said it would stop allowing investors to buy into or sell out of four private investment funds that rely exclusively on debtlike securities created by Greensill. A part of the funds is “currently subject to considerable uncertainties with respect to their accurate valuation,” according to a notice the bank sent to investors. The Wall Street Journal reported Sunday that the bank was concerned about Greensill’s exposure to a single client, U.K.-based steel magnate Sanjeev Gupta.