I also confirm what digitex said: There is no header dropdown, and the map container shows nothing. Looks to me as JS execution is stopped because of the following error:
Uncaught Error: Invalid LatLng object: (NaN, NaN)
LatLng https://unpkg.com/leaflet@0.7.7/dist/leaflet.js:6
latLng https://unpkg.com/leaflet@0.7.7/dist/leaflet.js:6
setView https://unpkg.com/leaflet@0.7.7/dist/leaflet.js:9
init https://www.xxxxx.at/site/modules/FieldtypeLeafletMapMarker/InputfieldLeafletMapMarker.js?v=3.0.3-1643876775:23
<anonymous> https://www.xxxxx.at/site/modules/FieldtypeLeafletMapMarker/InputfieldLeafletMapMarker.js?v=3.0.3-1643876775:179
jQuery 2
each
each
<anonymous> https://www.xxxxx.at/site/modules/FieldtypeLeafletMapMarker/InputfieldLeafletMapMarker.js?v=3.0.3-1643876775:177
jQuery 11
l
fireWith
ready
A
promise
ready
init
v
support
<anonymous>
<anonymous>
leaflet.js:6:13622
Using ProcessWire 3.0.184
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Maybe to be more specific: the module works like a charm, but in the backend it shows up without initial marker, and therefore no map is rendered. As soon as you type in any lat / lon the map shows up and you can place the marker accordingly.