Hello @Fabien55,
Does the calendar for the other ad show all Sundays as unavailable? That is, the date is grey with a line through it.
On the other ad, go to its "Availability settings".
Do you see "Not set" when you look at the "Check-in not allowed" line in the "Check-in and checkout" section? If it says "Sundays" then click it and remove the mark from the "Sundays" box and save it. If it says "Not set" and it's still impossible to arrive on Sundays then you should contact Airbnb support to discuss the issue.
Type "contact airbnb" in the "Search the community" box on these forum pages where you posted your question. Scroll down a little in the list of suggestions that appears and click on the "Contact Airbnb: A Community Help Guide[UPDATED]" one. That lists phone numbers and ways to contact them via Twitter or Facebook.
Steve.