"-- I load the Clean Vault Save and get an error message from DEF_UI (which as you can see I
don't have installed!) telling me I need to "make config XML". I assumed one of these mods relied on DEF_UI, which was causing the problem, but I've removed each one in turn and tried again -- same problem! Any suggestions?"
Yeah, I have seen that before.
If DEFUI has been removed then something in your LO has a patch for it. Like a patch you select when you install a mod with a built-in installer.
What can be hard to figure out is that patch may not show up in your LO and in-fact can be from a mod that has no esm/esp/esl.
When I had it, it was a mod in my mods tab. I had to disable everything in the mods tap not just the plug-ins tab to figure out which one it was.
At this point I suggest you disable all your mods in your mods tab.
Install SS2 according to this guide.
https://wiki.simsettlements2.com/en/getting-started/installation
After SS2 is working -
Then reinstall each one of your other mods to find the one with the installer that has the offending patch offered as a selection.
Lastly, when everything is working and if you feel you still want HUD/Inventory mgt mods then consider the FallUI mods. They are active and supported.
Sorry, after I replied I saw you had already responded and fixed it.
good deal!