I Bonds became one of the year’s most popular investments and one of the few financial havens as inflation reached a four-decade high and stocks plunged into a bear market.

As the deadline to get the 9.62% rate approached this week, the government’s TreasuryDirect site, the only place investors can directly purchase I Bonds, became one of the most visited federal websites, officials said, and has experienced intermittent outages for several days this week. 

Investors continue to run into difficulties accessing and logging on to the site.

“Due to unprecedented requests for new accounts, we can’t guarantee customers will be able to complete a purchase at the current 9.62% rate by the Oct. 28 deadline. The TreasuryDirect system has been and continues to process the payments that have been completed,” a spokesman said.

“We have tripled TreasuryDirect’s capacity in the last day and continue to see customers successfully create accounts and purchase bonds at record levels. Any additional updates to TreasuryDirect during the final days of the rate window, such as a delay to the Nov. 1 rate change, would pose significant risk to the operational integrity of the system,” said a Treasury spokesman.

If a customer receives a confirmation that their purchase has been made or completed by Oct. 28 11:59:59 p.m. ET, then the payment will be processed, a spokesman said.

Users regularly take to social media to complain about the TreasuryDirect website and sometimes go to great lengths to make their I Bond purchases.

“The TreasuryDirect website isn’t known for its user friendliness,” said Elliot Pepper, a financial planner in Baltimore. 

It isn’t just people trying to buy I Bonds who are frustrated with the site’s outages.

Investors can’t buy or redeem T-Bonds, Treasury notes, or T-bills through TreasuryDirect if they can’t access the site or log in due to the high demand.

Write to Veronica Dagher at Veronica.Dagher@wsj.com