Form not working (strange)

Hello,
I use this plugin from long time.

Normally everything goes well but I encounter a problem that I can not solve.
I set up a local site on which there are two research forms.
All works very well locally, both forms are in “custom search” mode and each returns to a page create with a specific template.
On the other hand, when I went into pre-production, one of the forms stopped working.
To check the parameters I created a third form with a single field that filters by categories.
This one does not work either:
– If I do not select anything in the drop down menu and I’m looking for I find the right items
– If I select a category where there are articles, it can not find anything
– And if I ask him to display the number of articles by category in the drop-down menu, he displays the correct number

I have checked the termplate, the settings several times and I am well at the indetique local and pre-prod.
An idea to solve this problem?