No Cal Invitations

Hello, i can´t invite people via e-mail to my appointments out of thunderbird/lightning. I can´t invite people with the android app davdroid.
I do create appointment, then enter adresses for invitation and save.
I expect that thunderbird created a mesage if i would like to send the invitation and then thinderbird sends invitation.
But: nothing happens after saving of new appointment, and no message is sent.
In the window for new appointament in “invitation” the field for “send invitation” is gray and can´t click.

Do you have any idea, how to resolve this?

There is already a longer bug report: