IB took losses on the oil contracts that were stuck. Positions could not be closed
" ... The price of the May 2020 contract dropped to an unprecedented negative price of $37.63. This price was the basis for determining the settlement price for cash-settled contracts traded on the CME Globex and also on a separate, expiring cash-settled futures contract listed on the Intercontinental Exchange Europe (“ICE Europe”).
Several Interactive Brokers LLC (“IBLLC”) customers held long positions in these CME and ICE Europe contracts, and as a result they incurred losses in excess of the equity in their accounts. IBLLC has fulfilled the firm’s required variation margin settlements with the respective clearinghouses on behalf of its customers. As a result, the Company has recognized an aggregate provisionary loss of approximately $88 million.
The Company does not believe that any anticipated losses will have a material effect on its financial condition."
Interactive Brokers Issues Statement on Crude Oil Contracts and Margin Loss