No map icons, blank tinymce and 500 error page

I have been doing some theme work for a client, and am experiencing a very strange problem with the plugin.

It started when I was editing a page, the tinymce editor was blank, if I switched to html view, I could still see my text there.
Trying to resolve the problem, I disabled all plugins and the tinymce worked again, one by one I reactivated the plugins and it turned out it was the leaflet map marker plugin that was causing the problem with the tinymce.

I solved the problem with tinymce by activating the plugin and then disabling the button in tinymce via the settings->misc section.

However I now also see, that on my site, the maps show no markers at all. If I turn on the feature to list markers under the map, they do all list there, but still not showing the icons on the map.

Also when I click the fullscreen button (or any of those buttons) all I get is an error 500 page.

I have tried
– going back to the older version of plugin (3.8)
– updating back to version 3.9 via WP update
– Manually uploading the files via ftp
– repairing the database tables
– disabling all other plugins
– switching to twenty fourteen template
– Trying different browsers
– Trying from a different computer

Site url: https://goo.gl/xSa7oB

Now I am stuck on what else to try to fix the problem, the map use to work fine.

https://www.ads-software.com/plugins/leaflet-maps-marker/