Microsoft says it’s investigating an issue in Windows that is causing application shortcuts in the Start menu or taskbar to disappear. Multiple IT admins have detailed the problem on Twitter and Reddit this morning, and it appears to be related to a recent update to the Microsoft Defender threat detections.

The problem is affecting businesses and organizations using Microsoft 365 and Defender for protection against malware, viruses, and other threats. In a note to customers, Microsoft says it has received reports that a certain attack surface reduction (ASR) rule is causing the problems. IT admins are currently trying to work around the issue by setting the “Block Win32 API calls from Office macro” rule to audit only.

Microsoft says it has now “reverted the rule to prevent further impact whilst we investigate further.” The software maker hasn’t issued a workaround or any guidance on how IT admins might recover the shortcuts on affected machines.

“This has caused a bad day,” says one IT admin on Twitter, replying to Microsoft’s acknowledgment of the issue. “Happy Friday 13th!” joked another. IT admins have been scrambling to fix the problem and bring the shortcuts back, with many having to tell their Windows users to manually launch their apps from where they’re installed or by using Start > Run and the executable name.

Three IT admins currently experiencing the problem at their organizations have confirmed to The Verge that all shortcuts have disappeared — and not just ones to Microsoft applications.

Regular Windows users and consumers aren’t affected by this strange bug, and it will only affect managed machines inside organizations. That’s still hundreds or thousands of machines inside big businesses that rely on Microsoft’s threat detection security.

We’ve reached out to Microsoft to comment on the situation, and we’ll update you accordingly.

Update, January 13th 8:20AM ET: Article updated with more information from Microsoft.