Users running Windows 11 report that the classic version of Outlook freezes after installing the September 2026 update. The issue appears to be a compatibility conflict with Kaspersky antivirus software.
According to reports, the application stops responding when syncing files from an IONOS email account. The problem occurs specifically when using the IMAP protocol to synchronize an inbox totaling approximately 500 MB, with the process stalling at around 50 MB.
Two workarounds are currently available for affected users. The first option is to uninstall the September update and revert to the August 2026 version of the software. Alternatively, users can fully disable Kaspersky Small Office Security to restore functionality.
