I have the same problem when the subject length is longer than 75 characters.
If the subject is longer than 75, the Email Class function _prep_q_encoding add an newline in the subject and then the problem appear.
I had the same problem as you. I changed the value if $crlf from “\r\n” to “\n” in the Email.php file in core/libraries and it seems to be working well now
var $newline = "\r\n";
var $crlf = "\n";