Skype SA says a software glitch caused the Internet calling and messaging service’s major outage last week.

In a blog post Wednesday, chief information officer Lars Rabbe said the 24-hour outage that cut service for millions of users stemmed from a problem in a version of Skype’s software for computers running Microsoft Corp.’s Windows operating system.

Rabbe said the issue, which began last Wednesday, started when some servers running offline instant messaging overloaded. An older version of Skype’s Windows software improperly processed delayed responses from these servers, crashing Skype for about 20 percent of users.

That crash included numerous “supernodes” — computers Skype likens to phone directories, helping users connect with each other — which resulted in a much larger outage.

On average, 124 million people use Skype each month.