Update. The problem seems to have been caused by using the updater to move from 5.0.7 to 5.5.1. I went into control panel and completely uninstalled PSPad (there were several instances from upgrading over the years) and then did a fresh install of 5.5.1 and it seems to work fine. The upgrade-in-place process (which is relatively new) may still have some issues.
-- <https://forum.pspad.com/read.php?4,79315,79316> PSPad freeware editor https://www.pspad.com
