Some Slack users were unable to access the popular collaboration app on Tuesday.

The outage was first reported by Slack at 9:25 a.m. ET as many US workers returned to work following the President's Day holiday weekend. Thousands of Slack users reported problems loading the app on DownDetector.com; the problems had not yet been fully resolved by noon ET.

"Slack is not loading for some users," the company had acknowledged at 10:23 a.m. ET on its service status page. "We are continuing to investigate the cause and will provide more information as soon as it's available."

At 12:07 p.m., Slack pointed to "signs of improvement. Please try reloading Slack, and if not a cache reset. We're still monitoring the situation. We'll confirm once this issue is fully resolved."

DownDetector.com shows the Slack outage.

For users still having issues as of about 1:45 p.m. ET, the company offered this advice: "We're continuing to see improvements, but we still have some work to do. If you're still encountering any trouble, please reload Slack using Command + Shift + R (Mac) or Ctrl + Shift + R (Windows/Linux) or clear the app cache (Help > Troubleshooting > Clear Cache and Restart on desktop, Preferences > Advanced > Reset Cache on mobile)."

The company finally reported at 2:24 pm ET that operations were back to normal.

Slack, which was acquired by Salesforce last year for$27.7 billion, has more 12 million daily active users. That's according to its most recently released stats from October 2019, so it's likely the number is considerably higher now.

While Software-as-a-Service (SaaS) vendors typically provide high levels of uptime, outages are not uncommon. Slack was one of a number of SaaS vendors knocked offline last December due to an outage at cloud provider Amazon Web Services' US East 1 data center apparently related to network device failure. Slack's services were also disrupted last September as a DNS configuration change created issues for some users. 

Slack did not immediately respond to questions seeking more information on the cause of the outage and how many users have been affected.

The Slack error messages some users saw.