According to Microsoft, Windows 10 and 11 users may experience a bug preventing Outlook and other apps from launching after installing the latest Patch Tuesday update (KB5027231, released on June 13, 2023). The company has updated its Windows Health Dashboard documentation to provide the necessary details about the problem and offer affected customers a workaround.
Microsoft says the bug affects 32-bit Office installed on 64-bit Windows 10 and 11 (client versions 22H2 and 21H2, plus Windows Server 2022). Even though Microsoft changed the default Office installation from 32-bit to 64-bit in 2018, many people may still be using 32-bit Office. However, you need a specific antivirus and a specific version to experience the problem—according to Microsoft, the issue occurs on systems using Trellix Endpoint Security version earlier than 35.31.25.
After installing KB5027231 or later updates on Windows devices with certain antivirus, antimalware or security software installed, you might not be able to open Microsoft or third-party apps. Some known affected apps are Microsoft Outlook and other Microsoft Office apps. This issue only occurs with 32-bit apps running on 64-bit Windows with Trellix Endpoint Security version earlier than 35.31.25. When attempting to open an affected app, Event Log indicates an access violation with the error "module is unknown". This issue is not likely to be encountered by consumers using Windows devices in their home.
Affected customers can resolve the problem by updating Trellix Endpoint Security Agent to version 35.31.25 and newer. Although Microsoft has only mentioned Trellix Endpoint Security Agent, other antivirus apps might cause the same problem. Therefore, the company recommends that those using other anti-malware solutions and experiencing issues with Office apps should contact customer support of the security software they use.
You can learn more about the problem with Outlook not opening on Windows 10 and 11 after the June Patch Tuesday update in the official Windows Health Dashboard documentation.