You're welcome : thanks for YOUR help(ssss).
I have some more info gave by Jisse on Joomla.fr
The problem here is not a problem of Flexicontent of Breezing Forms or Sh404.
Jisse told me that he found on some case the same situation : when you load several modules from different component on the same page, the problem occurs. It depends on the components, modules, etc.. used and called on the same page.
The solution found (and working here) is to load the module in an iframe in this case.
As he told me, he didn't find out why this occurs, but anyway, it's a working solution.
As you know, when you get a 500 error on a server, you are always glad : 500 error means "an error occurs but i can't give you more information, fend for yourself !", it helps very much !
I don't know if that's something to indicate to the Joomla Core Team as, again, it's not necessarly a Joomla problem.
Anyway, it's a piece of working solution
Thanks again to you men.
Best wishes