What did you change
$mail = \LimeMailer::getInstance(); to $mail = \LimeMailer::getInstance(\LimeMailer::ResetComplete);

Bad idea : no need to reset completly. Must fix differently.

I know, but this works for now. There must be a better solution of course.

OK : issue in $this->getIsHtml() checker …

It seems that phpMailer's '->ContentType' is reset. I think because it's a 'multipart' instead of 'text/html' after the presend or something like that.
The '->ContentType' has to be set again and might be done by inserting this after line 1454.
$mail->setSurvey($iSurveyId);

I try to understand when/why it resetted :)

We set it here
https://github.com/LimeSurvey/LimeSurvey/blob/6129c136b701ff07736b3ab7393fb407c3b369ef/application/core/LimeMailer.php#L246

Maybe at https://github.com/LimeSurvey/LimeSurvey/blob/6129c136b701ff07736b3ab7393fb407c3b369ef/application/core/LimeMailer.php#L227

Then
self::$instance->setSurveyId(self::$instance->surveyIs);

Fix it at 100%, but i want to understand why it's needed before apply the fix :)

Here I guess.
https://github.com/LimeSurvey/LimeSurvey/blob/6129c136b701ff07736b3ab7393fb407c3b369ef/application/third_party/phpmailer/src/PHPMailer.php#L1437

Yep, and there are potentially some other part resetted (header …)

You can check https://github.com/LimeSurvey/LimeSurvey/pull/1381

Thanks!

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=29481

Thanks, Denis!

Fixed in Release 4.1.1+200203