An error occurred while saving the commentMartin Scholz commented
I have that too every once in a while. My best guess is that it relays to the feature that - if you mark some part of an email before hitting "reply", it only includes the marked/highlighted part in the answer. So my workaround: if you mark all content of the original email (e.g. by "Cmd+A") before hitting "reply", the marked text is included.