Is it a New Record ?
Yes, it is a new record. All other fields can be entered, but the container one. It doesn't display it contents and nothing happens when you right click on it even when you are able to see it has the focus.
In layout mode you can usually see the name of each field inside the borders of the field. When you look at this in layout mode, do you see :: preceding the name of the field?
If so, your container field is from a related Tutorial: What are Table Occurrences? and the relationship between your layout's table occurrence and the field's table occurrence is not allowing you to enter data into the field.
No, I don't see the :: because I'm on the actual table, working on the actual layout but... for no reason everything's working now. Nothing changed, just worked. During the night the computer rebooted after some updates. Maybe that's the reason.
Anyway, thanks a lot for your posts!