I have carefully edited the invitation email a number of times, but clicking 'save' or 'save and close' makes it completely disappear from the email templates text box. When I go to send an invite, the email text box is blank. I sent an invite to myself to test whether or not it is actually blank, and it is indeed a completely blank and useless email. Why is this happening? Is there a character limit on invitation emails or something? I would have thought that my edits would not exceed a reasonable limit, it's a couple of short paragraphs and some word changes. Although I have not yet sent a reminder or confirmation email, those templates seem to be behaving normally - I can edit and save without it disappearing, and the text is present when I go to 'send' a reminder. What can I do to make the system recognise and save my invitation email?

Thanks

Which version of LS is used? What browser is used? Did you try to just save one character?
Perhaps a bug with your version/build of LS. Perhaps a issue with your browser or the content.

LS Version 2.67.2, same problem occurs in Firefox and Chrome. Saving one character seemed to work, saving one word also worked, but re-setting the whole template using the 'Reset This Template' button still disappears on saving, even when it has not been edited. Any advice? A blank invitation email (no survey link or anything) is pretty useless, so if I can't fix this I may have to abandon using Lime Survey, which would be a shame because everything else seems to work pretty well.

Thanks

Sounds like a bug to me. You might open a bug report. As a workaround you may check how much text the field can handle. Perhaps it is a certain character and not the length triggering the issue.
I personally never use LimeSurvey for invitations and cannot provide any "tricks".

I can not reproduce in french with/without html editor …

Use : www.limesurvey.org/manual/Debug_mode