Team17 support pinned a cause to the WD-L020 launch error that has been flooding reports since 22 September: a Windows 11 preview update, KB5124010. Posted by G_Medlin, tagged as a developer, on Steam Discussions: "a potential compatibility issue related to the latest Windows 11 update, KB5124010." Microsoft lists KB5124010 as a 22 September preview update for Windows 11 24H2 and 25H2 — not a routine security patch, which is why it has caught players who never touched the Windows Insider program.
The workaround Team17 posted: Start → Settings → Windows Update → Update history → Uninstall updates → find KB5124010 → Uninstall. They're explicit that removing a Windows update is optional and at your own risk, and that the team is "actively investigating" a fix that won't require it — if you'd rather not touch Windows Update, leaving it installed and waiting is the alternative they suggest.
This is a different, newer cause from the Windows Insider Preview build issue this site logged on 12 September — same error prefix, different trigger, and this one is now the confirmed one for anyone not on an Insider build.
Full WD-L troubleshooting page, all six codes and every fix reported so far →