Skype has attributed server overloads and a bug in Windows to its network failure last week that affected millions of users for two days before the Christmas.

“On Wednesday, December 22, a cluster of support servers responsible for offline instant messaging became overloaded. As a result of this overload, some Skype clients received delayed responses from the overloaded servers,” said chief information officer of Skype, Lars Rabbe on the company’s blog in an apologetic tune.

In a version of the Skype for Windows client (version 5.0.0152), the delayed responses from the overloaded servers were not properly processed, causing Windows clients running the affected version to crash, Rabbe said.

Around 50 percent of all Skype users globally ran the version of Skype for Windows, he noted.

By the time the company staff started responding to disable the overloaded servers a significant number of supernodes had already failed.

According to Skype, supernodes perform some of the administrative tasks in the network and make sure that calls get through.

The initial crashes happened just before our usual daily peak-hour and very shortly after the initial crash, which resulted in traffic to the supernodes that was about 100 times what would normally be expected at that time of day,” Rabbe added.

To avoid reoccurrence of such outage, Rabbe said that the company would examine its update policy, to allow users to automatically move to newer versions of the software.

We know that we fell short in both fulfilling your expectations and communicating with you during this incident,” Rabbe apologized.

The company also offered compensation for the outage by giving vouchers for pre-pay users and for subscribers, a free service for a week.