Have had the same problem here. in the server screen, up above, and in my case it apears the error messages on the pictures.
Life is Feudal Your Own crash a.png
Life is Feudal Your Own crash b.png
Life is Feudal Your Own crash c.png
It happens after I change the video settings to full screen and the grafics quality to high.
After that I could not login in my private server anymore, even if I could login in the ones on the web.
Tryed to uninstall and reinstall the game and delete and recriate an new private server, but have not solved anything.