Fujitsu phone support provided a solution. Suspecting that the ScanSnap home configuration file collection in my normal-use Mac account was somehow corrupted, they had me clear these files (under ~/Library/Application Support/FPU) away, forcing a reconfiguration. This worked, although with the minor consequence of losing scanner profiles.
From the Fujitsu support follow-on email
- Close out of ScanSnap Home
- Click the Apple icon and do a force quit and look for anything SanSnap and quit it.
- Click on finder icon in your dock tray.
- Click Go at the top of the screen and then select Go to Folder.
- Type ~/Library and click enter
- Then go to Application Support
- Rename the PFU folder to PFU Old
- Then relaunch ScanSnap Home and go through the scanner selection process