K2 Item view: Map stays gray

We sadly have a problem with the K2 Item detail view - the map stays gray, no map tiles are loaded.
The zoom Buttons on the top left are grayed out.
It is possible to drag the map, the map marker moves. But there are no map tiles visible - any advise?
There are no errors in the console.

interestingly, in the category overview the map tiles are visible and zooming works.

The website in question is in development, there is no easy direct link.
I can send you the IP for the host file if you like via email to see yourself,
please advise, thanks!
Jonathan

Please configure the plugin options to adjust zoom levels, the default values may not work in all situations.

What should I enter in the fields?
currently: std setting:
Map Zoom: -1
Maximum Zoom: -1
Minimum Zoom: -1
I have tried different numbers, but it had no effect so far.

Update: I think it is the -1 value, if no value is -1 it seems to work!

That's true.
The attached screenshot shows the working settings of the demo website.