“None of our lenders are able to submit loans right now,” said Blake Brock, the chief executive of Lendio, a marketplace for small-business loans. “We have billions in applications that are completely logjammed.”

The system had already experienced a slowdown on Friday, the first day of the program, Senator Marco Rubio, Republican of Florida and the chairman of the small-business committee, said in a tweet this weekend. According to one Washington bank lobbyist, one large bank reported that it took 72 minutes on Sunday night to enter one loan application and the E-Tran system crashed 13 times during the process.

But the outage may not have affected every lender. Frank Sorrentino, the chief executive of ConnectOne Bank, said his bank had been processing applications all day. He said if E-Tran had gone down, he had not heard about it, but his bank had experienced some slowness on the portal that they attributed to the volume of submissions.

ConnectOne has received more than 2,000 applications so far and has processed a “significant” portion of them, Mr. Sorrentino said.

The small-business agency is being closely watched for its ability to distribute the loans, which are meant to help businesses make payroll as the nation’s consumers are locked down to avoid being infected with the coronavirus.

An S.B.A. spokesman did not immediately respond to requests for comment.

Second U.S. company announces early trial of a potential coronavirus vaccine.

Inovio Pharmaceuticals announced Monday that it would begin a small safety test of a potential coronavirus vaccine in adults in Philadelphia and Kansas City, Mo.

Up to 40 healthy adult volunteers will get the Inovio vaccine as part of the first trial. Each volunteer will receive two doses of the vaccine — each dose four weeks apart — and the company expects to have initial results from the study by late summer.