I posted about it here in another thread after upgrading to 2.5.
It may be coincidence but when it occurs (it came back after a restart), HABpanel appears very unresponsive to the point of being almost unusable.
Restarting openHAB (after stopping and clearing caches) gives a different start up result every time. Sometimes all the ‘items’ in HABpanel become un-linked to their openHAB items. Sometimes the UIs do not appear in the dashboard.