Douglas spotted the issue and the contact form is now working properly.
I recommend this very useful extension.
I had been using this plugin on a Virtuemart site oput of the box!
This time I was using YOOlogin module with emailasusername plugin in header (horizontal position), and there was an issue with the input label. French string 'send' was displayed instead of Courriel (email).
Dylan helped me to solve this issue successfully, and with great patience.
I recommend this very useful plugin.
Support for the paid component is perfect.
Keep up the good work
Thanks for your review - just a slight correction: the Lite edition does not support recurring payment frequencies, only the standard edition does.