Can't get my Apple Watch to open my iMac

I'm running Sequoia 15.6 on a 27inch Intel iMac. This is the latest version. Likewise my Apple Watch OS has just been upgraded to WatchOS 11.6.

Before these upgrades, I could open the iMac by bringing my Apple Watch near. Now this doesn't work, despite my resetting the relevant properties. The iMac reports

"Your Mac was unable to communicate with your Apple Watch. Make sure your Apple Watch is unlocked, on your wrist and powered on" and refuses the connection.

What's wrong?