It might be a copy-paste issue.If you have created the message by some text redactor like WORD and transfer in to Electrum via copy-paste then It might be messed with reserved character. Try to put you message into Electrum by using keyboard.
this can not be a copy paste error!
as i said above your comment, the only problem here is in his signature byte length is invalid. and since the signature string is in base64 it could not have happened by accident. since accidental modification of base64 will most probably lead to invalid encoding!