Windows Defender issue

Last few days, Windows Defender (Win 10 Pro, 64-bit, v21H1) is having issues on my Inspiron lappy. It shows an alert on its icon in the notification area that a scan is due.

Open it and click Quick Scan and it runs for hours and hours and hours but never finishes. Left PC running overnight and it still hadn't completed by this morning. The number of files scanned counter was over 500K files this morning. The previously logged Quick Scan was ~2 weeks ago, took only ~4 min, and scanned ~57K files.

Ran chkdsk at boot and then DISM and sfc commands from CMD prompt and all were "successful".  Did a System Restore to an earlier date and that didn't help, even after rerunning DISM and sfc.

WU is still installing new viral signature files every day so WD is up-to-date. Just don't know what the scan is doing or why it won't finish.

BTW: There seems to be a bug in the WD Quick Scan tool. The screen says "Time remaining" but the number is increasing, not decreasing, so it's actually showing the time the scan has been running...

Anybody else had this problem with WD scans not finishing, and know how to fix it?