Can't transfer quote to invoice - June month problem in French

IP in French :
The transfer from quote to invoice doesn’t work for me when date of invoice is in June. I enter the invoice date and press the OK button and nothing happens. No error message

In the debug console I only see
/invoiceplane/index.php/quotes/ajax/quote_to_invoice HTTP/1.1 500 Internal Server Error 97 ms
and the application/logs/index.html is empty

When I simply change the date (language is french) from 07-Jui-2016 to 07-Jun-2016 it doesn’t work either since the system is converting 07-Jun-2016 to 07-Déc-2016.
BUT … if I choose 05-Jul-2016 it works !
And there is the same problem within an iinvoice : choosing an invoice date in June doesn’t work (the spinning reel displays)
A date in July is OK.

Turning IP into English solves (temporary) the problem since I need to return to French to issue the invoice.

My IP is on a remote server PHP Version 5.6.17-0+deb8u1


PS the select name “Enable the Debug Mode” is not visible on page (when IP language is french)

 <div class="form-group">
        <label class="control-label">
        <select name="settings[enable_debug]" class="input-sm form-control"
            <option value="0"
                Non            </option>
            <option value="1"
                Oui            </option>