Hi Greg,
As you know Module Custom is Joomla core one.
Check 3 things:
- If you have that folder:
modules\mod_custom - with all files inside, compare with Joomla_4.4.0-Stable-Full_Package.zip. In case of just override all files from that zip to be sure it's current.
- Is your template et_photography override that custom module, if yes - delete that override:
templates/et_photography/html/mod_custom
It's possible that used by you template was designed for J3, becuase was created 2 years ago. And mod_custom has a new Joomla 4 structure.
- Make template "Cassiopeia" default for 90sec and check if you still have that same error message.