I tried to add an entry by DOI.10.3109/17483107.2014.961179
JabRef 3.8 told me the "URL is invalid" but doesn't show me the URL.
There is a known issue with CrossRef that is fixed in the most recent development version of JabRef.
which then freezes ... also known ...
I just downloaded the newest version but still have this bug.
Just to make sure: Tobias was talking about the most recent development version of JabRef - which is available at https://builds.jabref.org/master - or you can use the beta release here: https://github.com/JabRef/jabref/releases/tag/v4.0-beta3. These versions should work - the newest stable release version 3.8.2 is broken in this regard.