Thank you for all your efforts you are putting in JabRef development. It seems to me that this snapshot does not have the failings of earlier ones. I have been adding entrys from DOI, medline via PMID, shuffled the entries into groups, without any problem. Editing entries also works nicely.
The only thing twisting my nose is that the Ctrl-N dialogue is in the upper corner of the screen, not in the upper corner or in the center of the Jabref window. But in think that is a minor issue.
Since it seems that changing the Theme of the display leads to different error, I am not experiencing with that. But I hope when the issue is solved I can come back to something which is not as white as the actual screen.