Geo Mashup map does not show

I have two sites (WP 5.4.2) running the plugins:
– ‘Geo Mashup’
– ‘Async JavaScript’
– ‘Autoptimize’

Problem is the map is showing a grey color and does not load anything. tested in Chrome and Safari and on Safari mobile.

If I disable the ‘Async JavaScript’ plugin, all is fine. I tried excluding jQuery, I tried excluding the ‘Geo Mashup’ plugin, but nothing worked.

So to compare, two (almost) the same websites, with the same plugins:
https://www.urbex.nl/map/ (‘Async JavaScript’ enabled, grey block)
https://www.chernobyl.one/chernobyl-map/ (‘Async JavaScript’ disabled, map works)

Any help? Thanks.

(Most strange is, sometimes the map actually works even with the plugin enabled, but after refresh is almost always a grey block again.)

The page I need help with: [log in to see the link]