406 error when saving

Hello,

I have an issue using the plugin Contact Form on the website I’m developing. I created a personalized theme (url: http://ce-labadille.com) but I don’t think the issue come from there.

The error at first occured when I tried do save/apply form, it didn’t save it and take me back in the form list. The thing odd is each time I tried it, it duplicate /admin.php in the url like this (for a 3 time edit):
http://ce-labadille.com/wp-admin/admin.php/admin.php/admin.php/?page=manage_fmc

Then I tried to change the theme (same result)
To disable all other plugin (same result)
From there I try to solve the API key missing value error thinking it was maybe the solution
After solving this js error, i try again and this time get a:

Not Acceptable

An appropriate representation of the requested resource /wp-admin/admin.php could not be found on this server.

Additionally, a 406 Not Acceptable error was encountered while trying to use an ErrorDocument to handle the request.

I do some research and found a lot of stuff about htaccess solution:
put this code in .htaccess wp-admin root:

<IfModule mod_security.c>
SecFilterEngine Off
SecFilterScanPOST Off
</IfModule>

But it did nothing so i try another one in my public_html directory:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

It solved the 406 issue…. BUT…
I get exactly the same bug than before everything i’ve done (url issue /admin.php/admin.php)

Thanks for your help, it’s been douzains of hour of debug and nothing changes -_-…