This IS a WP pop-up, may I ask why you thought it wasn’t?
The issue will most likely be caused by 2 things:
1. Check your console log for the AJAX data recieved when uploading the file. Is there an error in the returned AJAX data?
2. Are you using a user account which is not administrator? If so, you must have the correct permissions for the user to upload an image. There are a lot of threads about this on the net but unfortunately I haven’t written a doc about it.
Let me know what you find.
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