When replying to a message which has the header line
# Content-Transfer-Encoding: quoted-printable
then the setting
"Indent the original text with " " when replying or forwarding"
gets NOT applied.

Ie. this is a bug in OE.

For example the poster "...winston" ) has
the MIME setting set to "quoted printable" and this error
happens when replying to his messages.
He should set MIME to "None" in Options/Send/PlainText/PlainText Settings.