Zoom has stopped working, leaving at least some users unable to start or join video meetings.
The company's website is also failing to load, it said.
Zoom has become perhaps the most significant video calling and conferencing app amid global lockdowns, allowing people to both work and socialise despite orders to stay at home.
Zoom said on its service status page that it was aware of the problems and was investigating.
"We have received reports of users being unable to visit the Zoom website and unable to start and join Zoom Meetings and Webinars. We are currently investigating and will provide updates as we have them."
A spokesperson said in a statement: "We sincerely apologise for any inconvenience.”
Issues were particularly focused on the east coast of the US, according to tracking website Down Detector. But it is not clear if that is simply a result of where most people are trying to get online, or if the issues are particularly geographically focused.
As it became popular in the early days of lockdown, Zoom was hit by a number of security, privacy and reliability problems.
That led the company's founder and chief executive, Eric Yuan, to commit to a 90-day plan, during which it improved a host of privacy settings and added new tools for keeping calls secure.
As he launched that plan, Mr Yuan admitted that the company had struggled to cope with the growth of new users and the extra attention that brought.
In recent times, that work has been completed, and the company has rolled out new features. That has included new face filters, as well as ways to "touch" up both audio and video.
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies