Hi @ johnm, have you tried to remove the old path like this:
- On the child site go to the ../wp-admin/options.php page, it will show you all wp_options database table options.
- Use the Ctr + F to find the following string: upload_path
- Delete the value from the field
- Scroll all the way to the bottom and save changes
This should fix the problem.
The other maybe easier way to do it is this:
Go to the Settings > Media and delete teh Store uploads in this folder field. Leave it blank and save changes.
Let me know how it goes