Microsoft has apologized for a lengthy Skype outage on Monday, blaming the issue on a configuration change which apparently disconnected many consumer versions of Skype from the network.

On Monday, Skype was reportedly down or suffered service trouble for about 15 hours, according to reports.

Microsoft acknowledged the problem on Monday on its Heartbeat blog, finally claiming at around 5 PM Pacific time that the problem had been resolved. That was about ten hours or so before Microsoft began rolling out Office 2016 to users, although the company said that Skype for Business was not affected.

Why this matters:  For many, Skype is akin to a utility, the communication medium for small teams and business contacts alike. Yes, Skype for Business never went down, but as more and more people use their phones to reamin in contact 24/7, the difference between Skype for Business and the consumer version of Skype becomes somewhat irrelevant. Microsoft’s networking teams may have been focused on the Skype for Business rollout, but they’ll need to do better next time.

What did happen, according to Microsoft, is that a larger than normal configuration change was pushed to some Skype clients—which, for whatever reason, couldn’t process it and were disconnected. When they came back online again, according to Skype, “heavy traffic was created and some of you were unable to use Skype’s free services, including messaging, presence, and contact list management,” the company said. Others could not sign in or out of Skype altogether, or make calls to landlines or mobile phones. 

The additional traffic burdened the system, preventing many from using the company’s services. The solution, according to Microsoft, was to rebalance the load that the outage put on the system, so that existing users could continue to use the Skype services. 

That sounds like the type of outage Skype suffered in 2007, when the network went down after millions of computers rebooted themselves after a Windows update. Skype traffic flowed through certain computers or “supernodes,” and when those computers went down, so did Skype. A similar problem took down an estimated two-thirds of Skype users in 2010. 

Microsoft also apologized for the outage. “We are extremely sorry for any inconvenience caused to our users, and appreciate your patience while we addressed the issue,” the company said.