In some cases it happens for me (I use Evolution extensively) I can’t add a signature or picture, but can write.
When i send the mail, the content will not be send - blank mail.
After restart Evolution, all is fine again.
I have made a service thread, but maybe are not the only one seen that issue?
http://gnome-evolution-general.1774414.n4.nabble.com/empty-message-content-td4669989.html
https://gitlab.gnome.org/GNOME/evolution/issues/587
I found this here and updated webkit2gtk. We will see if it works…
I updated webkit2gtk to a newer version and it seems so far to be fine now.
This might be the solution.
As I can see, this problem had been before several times. Maybe Evolution tam should have a eye on this issue in future?