(Reuters) — Video-conferencing company Zoom Video Communications Inc. said on Monday it had restored service to its U.S. users after a partial outage left many unable to log in to work meetings or attend school classes remotely.

The San Jose, California-based company has experienced a surge in usage during the coronavirus pandemic, as millions of people turn to it for work meetings, school, social events including weddings and to otherwise stay connected while isolating themselves. Many schools that turned to remote instruction have used Zoom for classes.

Outage tracking website Downdetector.com showed nearly 17,000 incidents of people reporting issues with Zoom earlier in the day.

“We have resolved the issue causing users to be unable to start and join Zoom Meetings and Webinars. Users are now also able to sign up for paid accounts, upgrade and manage their service on the Zoom website. We are currently monitoring to ensure that these services are operational,” Zoom said on its website.

The company did not provide details on the cause of outage.

Zoom’s stock has risen more than eight-fold since its initial public offering last year and four-fold so far in 2020, but it was down 2.9% after falling as much as 5.4% in earlier Monday.

Zoom competes with Cisco Systems Inc.’s Webex, Microsoft Corp.’s Teams and Alphabet Inc.’s Google Meet platform for paying customers, particularly enterprises, while offering a free version to customers.

Zoom had 300 million daily meeting participants in April, the latest figures disclosed.

Even as its usage has soared, Zoom has come under fire over privacy and security issues, including incidents of “Zoom bombing” in which uninvited users entered and disrupted meetings. It has since rolled out major upgrades, including end-to-end encryption for video calls.

Although a California company, Zoom has big research and development centers in China with hundreds of employees, according to a filing it made to the U.S. government.

“For sustained growth to continue, it (Zoom) will have to show investors that it can be relied on to ensure its core customers don’t drift towards the likes of Microsoft Teams, Google Meet and Cisco’s Webex,” said Susannah Streeter, senior investment and markets commentator at Hargreaves Lansdown.

“As Zoom fatigue sets in and the pandemic eases, it’s even more important that the company demonstrates its systems are secure, if it’s to be seen as a long-term player in this competitive market,” Streeter added.

Users took to Twitter to complain about the outage.

“Zoom having a world wide outage for the first day of school, so 2020!" Twitter user Anthony Slaughter wrote.