I’m having the same issue with 2.38.5 on one of my sites, but it’s working normally on the other.
Both “Upload Path” and “Fileupload URL” are pointing to the same directory. There was no “mappress/icons” directory (I deactivated and reactivated Mappress in case it was something created on activation), so I created a new one and put the icon in there.
Each site is on a different multisite installation and the plugin is only activated on a site-by-site basis (not netwok activation).