I will try to describe what happens.
If I turn off all Sef (Joomla! and JoomSEF), the link in the menu to the Flexi Contact page is:
mysite/index.php?option=com_flexicontact&Itemid=9&lang=fr
In the <form> in the page source the action is:
<form action=/mysite/index.php?option=com_flexicontact&Itemid=9&lang=fr">
Turn on only Joomla! SEF and the link in the menu is:
In the <form> I see the following:
<form action="/mysite/fr/flexi">
Turn on JoomSEF and 'Don't SEF' Flexi Contact, the link in the menu is:
mysite/index.php?Itemid=9&option=com_flexicontact&lang=fr
In the <form> I see the following:
<form action="/mysite/index.php">
In the last case the form doesn't work any more.
I cleaned the cache, purged the URLs, cleaned the cache of the browser. Nothing helps.
Somehow it looks as if JoomSEF is doing something to the Flexi Contact, but I haven't got a clue what.
Hopefully you can help.
Marga