I think your coordinates is the problem. Try to change this --> "coordinates":[[[[10,30],[30,30],[30,10],[10,10],[10,30]]]] to this --> "coordinates":[[[[10,30],[30,30],[30,60],[10,10],[10,30]]]]. Consider that to shape a polygon you have to respect the right hand rule.

QGIS embeds map layers on a desktop app , not on the web. So some layers if they are not CORS enabled will not work when exposed on the web. For OSM I can guaranty it will work, but to fix any other layer you need access to the server generating those layes.

You may have basemaps that do not allow exporting. For example I know that OSM is capable to export while google maps not. If you get basemaps from local geoserver propably it is not CORS enabled. Try to see if you can execute the export while having in your map only OSM as a basemap.

What sort of layers do u have on your map when you execute the export?

Its true, there are users complaining about this case. However, you may cache the styles and avoid this performance issue. check this --> stackoverflow.com/questions/40796548/… and this --> openlayersbook.github.io/ch06-styling-vector-layers/…

@wondim Thats why I am suggesting the solution above. In order to avoid style reasign for each feature on each pan.