Create & Send new message: Same but different.
I get "An error occured while saving", the message remains on screen and is NOT moved to the sent folder.
But the message is sent anyway, which can be confirmed by logging into the recipient address.
The sending userid just gets the above message message at the top of the screen and when you exit the message that remains on screen a window pops up saying "message has not been sent, do you want to discard your message"
This problem is, in my case, with all the email userid's that existed before Roundcube was installed. The one userid setup AFTER the Roundcube install works OK and gets a confirmation message at the top and the message is moved to the sent folder.