Medline is currently changing its infrastructure which also affects JabRef (more technical details here…). We have fixed the Medline fetcher in current development builds and are currently in the process of releasing JabRef 3.7
So, if you don’t want to wait and to test it out you can use a development build which is available here: https://builds.jabref.org/master
Or you wait until the end of the week - than the official release will take place (if no serious problems are reported before).