The U.S. Securities and Exchange Commission has issued warnings to issuers of high-leveraged exchange-traded funds, including proposed crypto ETFs, citing concerns over potential risks of such products.  In letters sent Tuesday, the regulator halted the review of the funds until concerns were addressed by the issuers. The SEC reviews applications for ETFs before they start trading on stock exchanges.  The regulator warned a total of nine issuers, including ProShares, which already has leveraged crypto ETFs ...