"Sorry, I could not acquire root privileges" error message on Titanium Backup

When running Titanium Backup, it fails with Sorry, I could not acquire root privileges [...]. The su it reports as using is /system/bin/su - it exists and works. My environment: Google Pixel 8 Pro running the latest version of the standard Husky rooted with Magisk. I have tried all the usual stuff: reboot; uninstall and reinstall TB; and a host of other useless methods. After the reinstall, Magisk asks whether to grant root privileges to TB. Subsequent starts of TB result in a Magisk toast saying that root has been granted to TB. So TB actually has root privileges but doesn't know about it. This is REALLY annoying as the P8 is new - replaces a P5 so I need to restore a few things.