No idea… Maybe move this discussion to a more general openHAB 2 category since this is likely a problem with your installation, not with HABPanel.
You should at least be able to go to Paper UI, Configuration > Services > Configure HABPanel and Save and it should work.
One more thing you can try:
Assuming you have the REST API documentation site installed, go to http://yourip:port/doc/index.html#!/services/getConfiguration
put org.openhab.ui.habpanel as serviceId, hit “try it out!” , backup the results just in case then go to
http://yourip:port/doc/index.html#!/services/updateConfiguration
and paste the response body in the body text area. You’ll probably have your 500 error, hopefully with more details.