JabRef 4.3.1
Linux 5.0.0-27-generic amd64
Java 1.8.0_222

I have not had this problem with the v.5 test releases.

Steps to reproduce the behavior:

1. Open entry editor,
2. select "General" tab
3. click the + sign to add a PDF file
4. The file-picker opens (Nemo for me, since I'm in Linuxmint/Cinnamon)
5. type the first letter of a filename to jump to that file
6. Either system freezes for several minutes until a timeout, or Jabref and immediately nemo crash.

Sorry, can't give you a log because of crashes.
The problem is intermittent but common.