Hi there,
My apologies for the issue you are experiencing.
When the plugin doesn’t save and render custom markers (assuming that the normal markers do render correctly), the most likely reason is that the plugin doesn’t have the necessary folder permissions.
Please could you check the folder permissions on the plugins or wp-content folders in your site. It’s very possible that PHP (which operates the plugin backend) doesn’t have permission to add files to sub-folders of one of these folders, which would explain the issue. Depending on the operating system of the server hosting your WordPress site, there are different ways of giving write and read permission to PHP.
https://stackoverflow.com/questions/2900690/how-do-i-give-php-write-access-to-a-directory
Please let me know if you come right, or if you need any further assistance in this regard.
Kind regards,
Steven