@yuriv I take it back. I got some other free versions working so premium had nothing to do with it, it was just a coincidence. Sorry for jumping to conclusions!
After some initial panic I have now updated and checked seven different installations of MailPoet 2 on WP 4.9 and only one (of course the first one I updated and really needed to use right away) was broken.
After ripping out all the other plugins I had on that one site I found that it was another plugin that kept the editing sidebar from loading. Without it MailPoet 2 works as it should again.
