Minutes after a $310 billion aid program for small companies opened for business on Monday, the online portal for submitting applications crashed. And it kept crashing all day, much to the frustration of bankers around the country who were trying — and failing — to apply on behalf of desperate clients.

Some bankers were so irritated that they vented on social media at the Small Business Administration, which is running the program. Rob Nichols, the chief executive of the American Bankers Association, wrote on Twitter that the trade group’s members were “deeply frustrated” at their inability to access the system. Until the problems were fixed, he said, “#AmericasBanks will not be able to help more struggling small businesses.”

Pent-up demand for the funds has been intense, after the program’s initial $342 billion funding ran out in under two weeks, stranding hundreds of thousands of applicants whose loans did not get processed. Last week, Congress approved the additional $310 billion for small businesses hit by the coronavirus pandemic. Bankers were expecting the money to once again run out quickly, and so on Monday at 10:30 a.m., when round two opened, they were ready to go.

But for the second time in a month, the relief effort, called the Paycheck Protection Program, turned into chaos, sowing confusion among lenders and borrowers. A centerpiece of the government’s $2 trillion economic stimulus package, the program offers small companies — typically those with up to 500 workers — forgivable loans of up to $10 million. The S.B.A. is backing the loans, but customers must apply through financial institutions.