At JabRef > Options > Preferences > Entry Preview
If you move more than a certain number of entries from the Available box to the Selected box, you get an error as shown in the attached image.
In my version of JabRef (JabRef 5.3–2021-07-05–50c96a2, Windows 7 6.1 amd64, Java 16.0.1, JavaFX 16+8 ), this bug appears after having moved all entries from the Available box, up to the entry Australian Veterinary Journal ; then, when moving the following entry Australian Journal of Development Studies… the problem illustrated in the attached image appears.
A feature that would be nice in JabRef is a preview of citation styles. That is why I tried to move all Available entries to the Selected box.