Slack, the workplace chat app, has stopped working properly.
Some users reported they were unable to get online or to send messages to their colleagues.
But others were entirely unaffected, with the app continuing to work as normal.
Slack claimed that the problems were affecting only 1 per cent of its users.
The issue appears to be with DNS, or Domain Name System, which is one of the technologies that underpins the internet.
Slack said that it had fixed the underlying issue, but that because of the distributed nature of the internet it could take some time to start working for everyone.
In the meantime, the company suggested a number of possible workarounds. It might be possible to reload the app – by pressing control or command and “R” – and it might be possible to use Google’s DNS system.
Slack noted, however, that many people will be using its app on work computer systems, which often include restrictive rules that ban users from changing such settings.
The problems ran overnight UK time, with Slack recognising the outage just after 5pm in the UK. It has said since that it seeing signs of improvement, as the fix rolls out to users, but recognised that the problems were continuing.