[Theme: Hum] Problem with the wp Loop in a twenty eleven child theme?

Hello everybody here,

Trying to start a new WP site, I’m getting this trouble : On the front page the posts are properly displayed but in single posts the text goes further than the main field.
I already tried to disable the plugins but it did not help.

The design of this theme fits good to want I want to do, so I’d like to keep it, but I have no idea at all of how to resolve the problem.

Any idea?
Many thanks for the help.