This is the position/ case number of the repeat loop.
This user might use a special browser or a special language configuration or an older form url link.
Can you try, which error messages you can reproduce yourself?
Do you use multiple languages in this form? There might be an error message (e.g. constraint_message) not defined for the used form language.
The group label message is just a warning. Without label there is no visible group “title”. See Error when trying to preview - #4 by wroos.
