Thanks for the quick reply - fresh install produced no change.
Further research on the error message lead me to look for plugins of type editors-xtd and I found one from pre-joomla 4 upgrade that had not been updated/disabled. Disabling that button's plugin, the New Category page renders with the error:
Warning: Undefined property: Joomla\CMS\Object\CMSObject::$name in C:\xampp-824\htdocs\plugins\editors\tinymce\src\PluginTraits\XTDButtons.php on line 61
I tested the editor-xtd plugins one at a time and isolated it to the Editors-xtd - T4 Page Builder plugin.
Turning off error display hides the warning, but the devs should update the code. I don't know if not initializing $name has any effect other than being an unexpected exception for PHP that probably triggers a default to null.
I'm continuing on.