Sending email hang with html format checked

I have a form where I would to use html format in email for changing the color of some text .

All work well with the HTML case not checked.

But if I just check the box to send the mail in html format, without using any html formatting in the email, the form freezes and the little wheel spins and spins endlessly. The email is never sent!

My version of CF7 is 6.0

Thank you for any advice!

Mario