i am using a Docker Nodered instance in combination with Hassio .. in that case all the NodeRed Entpoints get prefixed with /endpoint/ … so i had to change
line 545 in 77-openhab2.html from
$.getJSON("/openhab3/items", config)
to $.getJSON("/endpoint/openhab3/items", config)
Hope this helps someone else doing the same thing and facing the same problem !