Habpanel: only a black screen is shown after first mod

till now I use the basicui view to see my configured item in oH. Works, but not very nice.

Now I try to make the view more customized, install habpanel and make first steps (add a first own "dumme (temprature)).
I think I try to change the “Format” in the habpanel GUI for my own temprature item (item was shown before with 21.5376374); and now, the screen is black. Nothing; no hyperlink, no setting, no scroll-bar.

Address I use is: http://MY-IP:8080/habpanel/index.html#/
(where is this (corrupted) index.html located on Raspberry or the config-file which contains my added “dummy” widget with corrupt data/parameter … ?)

I allready de- and re-install the userinterface “habpanel”, restart oH services, but still: black.
The basicui is ruinning as before.

And ideas? Is there a (corrupt) config file I have to delete?
Where are the (modified) files from the habpanel physically stored on the Raspberry filesystem?

Add. info / update:
when re-install the habpanel and pressing continuously “F5” on the browser, for 2 seconds the (default) screen is coming (with Livingroom, Garden, …), than it’s going whole black!

(Seem same issue as here, but should be fixed … )

I make a:
#openhab-cli restore xxx.zip from 3 days ago => also black screen !

On this version the hab-panel was untouched and working fine!
Where can be the problem?

No one a hint? Re-install whole system?