Explorer crash - ucrtbase.dll

The event shows that sihost.exe, rather than File Explorer itself, terminated while calling ucrtbase.dll with exception 0xc0000409. That identifies the crash location, but it does not prove ucrtbase.dll is the root cause. System-file corruption or a background software conflict still needs to be tested. Since DISM and CHKDSK have already completed, install all Windows updates, including any offered Lenovo driver updates, restart, then run an elevated Command Prompt and execute sfc /scannow. Note whether SFC repairs anything. If the crash returns, perform a clean boot by hiding Microsoft services, disabling the remaining services and startup apps, and testing normally. Re-enable items in groups to isolate the conflict. If it still crashes in a clean boot, press Win+F after the next occurrence and submit Feedback Hub diagnostics with the Event Viewer entry and report ID. Avoid replacing ucrtbase.dll manually; use Get Help or Microsoft Support if SFC cannot repair files.