Not tested this, but could it be that also the php version was upgraded and this might be the cause?What's weird is that it has always worked perfectly. I am using the same email template as before and now it doesn't work.
Our mwz installs work the plain & html combined per email,just checked one for you. PHP version is 7.2, but it worked for us all along, since many versions of mwz and php 7, 7.1, 7.2.I don't know if PHP was upgraded. I have PHP 5.6 (ea-php56) on the server. Is there a specific version I need for it to work?
Thank you.