Can't send more than one message

[mgdb]

Hello,
My problem is :

I send my first message. It is OK.

When I want send another message, I have the "the message can't be delivered"

To resolve the problem, I must quit roundcube and login again.

Have I miss something ?
Thanks for your help

[mgdb]

Hello,
I reply to myself :
When I put a signature, format text or html, I can't send more than one message successively.
If I delete the signature, I can send a more than one message successively.
Is it a bug ?