First xsane told me that it couldn't find any devices. I reinstalled all the hplip and xsane packages, and that got me a message that xsane couldn't open the device -- giving a name that was obvious that it knew about my scanner.
I found a message in /var/log/syslog that xsane couldn't find the file "/usr/share/hplip/scan/plugins/bb_soapht.so". So I ran:
sudo hp-pluginand answered the question about license. Then xsane (and the new Simple Scan) worked.