On September 22, 2026, Microsoft released the optional Preview Update KB5124010 for Windows 11 versions 24H2, 25H2, and 26H2. This update is designed to enhance performance and address various issues, with broader distribution planned for the October 2026 Patch Day. However, feedback from users has raised concerns about potential complications arising from this update.
Preview-Update KB5124010
The Preview Update KB5124010 not only supports Windows 11 versions 24H2 and 25H2 but also aligns with the release of Windows 11 26H2 on September 29, 2026. This update introduces a range of improvements and fixes, which have been detailed in previous discussions regarding Windows 11.
Reader Reports Issues with the Preview Update
A blog reader, Christoph R., reached out on September 30, 2026, to report issues encountered within his company following the installation of the Preview Update. His insights shed light on specific problems affecting software functionality.
Crashes Related to msmpeg2ac3dec.dll
In his correspondence, Christoph highlighted that several applications were crashing due to issues with the library file msmpeg2ac3dec.dll when multiple MP3 files were played simultaneously. This file is a standard component of Windows, located in:
%WINDIR%System32 %WINDIR%SysWOW64
Notably, older games such as SimCity 4 and Fallout New Vegas have been reported as particularly vulnerable to these crashes. Christoph pointed out that discussions on platforms like Reddit reveal a wider community experiencing similar issues, with threads dedicated to the problematic update.
Telephone System Client Also Affected
Christoph further noted that their company’s telephone system, powered by innovaphone, was experiencing similar crashes with the myApps client, which includes their softphone functionality. He mentioned that there has been no response from the manufacturer regarding this issue, prompting internal advisories to uninstall the update until a solution is found.
Interestingly, Christoph discovered that the msmpeg2ac3dec.dll file is not present on all Windows 11 systems. His internal research revealed that the file appears only on systems upgraded from Windows 11 22H2, while fresh installations of versions 24H2 or 25H2 do not include it, allowing the telephone client to function without issues.
A Workaround via LAV Filters
In his initial email, Christoph mentioned a potential workaround he found on Reddit involving the installation of “LAV Filters” from their GitHub page. These filters, based on ffmpeg, serve as DirectShow splitters and decoders, effectively isolating media data from the problematic DLL.
In a follow-up message dated October 1, 2026, Christoph confirmed that he tested the LAV Filters and found them to resolve the crashing issue without requiring any special configuration during installation. He noted that while he could not fully assess the broader implications of this solution, it was deemed non-critical for their operations since the client primarily handles telephony tasks, isolated from the internal network.
Confirmation from Microsoft
Update: Microsoft has acknowledged the issues associated with the Preview Update KB5124010 in their support documentation under Known Issues. They reported:
Applications which require AC-3 audio decoding might close unexpectedly. Symptoms: After installing this update, some applications that use AC-3 (Dolby Digital) audio decoding might fail to launch or close unexpectedly when certain audio features are used. This issue primarily affects legacy applications that rely on Windows built-in audio decoding. Most modern applications are not affected. Resolution: Microsoft is working on releasing a resolution for this issue in a future Windows update. We will provide an update when more information is available.
Related Articles:
Windows 11 24H2 – 25H2: Preview Update KB5124010 (September 22, 2026)
Windows: Bug in File History Confirmed and Fixed by September 2026 Updates
