Ok, I use qmt plugin as a list and not as a dropdown. By doing so, I can filter serveral Times my search query. When submitting the form for filtering my custom fields I Get different results depending on the search I've done before:
Case 1: selection of one category from the qmt list --> by Submitting the custom fields form I get a good result.
Case 1.1: by selecting another category from the qmt list, the custom fields form I had before disappears.
Case 2: selection of 2 categories from the qmt list --> by Submitting the custom fields form I get an error result (the qmt filtrers disappear)
Many thanks and I hope we'll find a solution :)