I found the solution and I give it there if somebody have one day the same problem.
For a reason X, the system didn't wanted to send a picture was into my signature. This picture was a link from my internet site.
I removed this pic and I download one (the same) from Roundcube directly and problem solve.
Don't asked me why, but it's working now.
Just, I can't add any domain links into the signature like before, but it's totaly ok like that (link on the pic will be maybe an idea...).
Have a great day!