Can't save settings in backend

Installed JEM 2.3.5 in my development host. No problem changing settings and saving in the backend.

So then I installed the same on my production host, and I can't save Settings:

Attempt to assign property "css_backend_usecustom" on string

The 2 hosts should be the same but php in dev is 8.2.18 but only 8.0.30 in prod.

I turned on debug to get this:
1       ()      JROOT/libraries/vendor/joomla/registry/src/:814
2       Joomla\Registry\Registry->bindData()    JROOT/libraries/vendor/joomla/registry/src/:809
3       Joomla\Registry\Registry->bindData()    JROOT/libraries/vendor/joomla/registry/src/:335
4       Joomla\Registry\Registry->loadObject()  JROOT/components/com_jem/classes/:150
5       JemConfig->bind()       JROOT/administrator/components/com_jem/models/:95
6       JemModelSettings->store()       JROOT/administrator/components/com_jem/controllers/:121
7       JemControllerSettings->save()   JROOT/libraries/src/MVC/Controller/:702
8       Joomla\CMS\MVC\Controller\BaseController->execute()     JROOT/administrator/components/com_jem/:48

(added)
OK, I see css is defined in #__jem_config in dev but is missing in my prod db. Not sure how to fix that or how that could happen (from a fresh initial install)

(sigh)
I saw the older post referring to that variable, so I ran the UPDATE to add in the value of CSS. That fixed the error.
 

OK, but why JEM 2.3.5? – are still on Joomla 3?
Please migrate to Joomla 5, at least for securit reasons.

Yes, still on Joomla 3. It's embarrassing but I made so many customizations across core and a dozen extensions, I am still trying to migrate.