Here are some warnings regarding your setup: Need help

I’m getting the following error in my setup "it’s been there since NextBox was turned on.

  • Some app directories are owned by a different user than the web server one. This may be the case if apps have been installed manually. Check the permissions of the following app directories:
  • /var/www/html/custom_apps/nextbox

Is this an issue? If so, how can I fix it?

That’s intended and covered in the FAQs:

Why am I getting a permission warning for /var/www/html/custom_apps/nextbox inside the Nextcloud settings overview?
This is a “feature”. The NextBox Nextcloud App is installed on the system with the Debian nextbox package. To avoid an accidental deletion of the NextBox Nextcloud App from within the Nextcloud app management, the stated directory can not be written by Nextcloud, this is what Nextcloud is complaining about here.