LONDON/NEW YORK—British bank Barclays said on Monday it had suspended the sales and issuance of two exchange-traded notes (ETNs) with combined assets of about $1 billion—one linked to crude oil and another to a gauge of market volatility—due to capacity constraints, in a move that some investors said could spur big price swings in the products. ETNs are debt securities that banks issue with the promise to pay holders a return linked to the performance of underlying securities or benchmarks. Investors are particularly sensitive about the product ever since another volatility-tracking ETN called XIV went bust in a matter of days in February 2018, dealing nearly $2 billion in losses to shareholders. The ETNs affected are called iPath® Pure Beta Crude Oil ETN and iPath Series B S&P 500 VIX Short-Term Futures ETN. Barclays, which expects to reopen sales and issuances of the ETNs as soon as it can accommodate …