Experienced similar behavior when updated to 2.5 Snapshot #1501. BasicUI complained about server being offline. This line showed up in the logs always when the page was refreshed.
2019-01-16 08:33:16.520 [ERROR] [e.internal.WriterInterceptorExecutor] - MessageBodyWriter not found for media type=text/event-stream, type=class org.glassfish.jersey.media.sse.OutboundEvent, genericType=class org.glassfish.jersey.media.sse.OutboundEvent.
I didn’t do a restart after upgrading and noticed this some time after the upgrade process. Can’t say if solely a restart would have done the job but I stopped oh, deleted /var/lib/openhab2/config/org/openhab/addons.config and started oh again. Problem solved.