Marker overlays must be png, jpg, or webp

What is wrong with my Overlay?  Here is example url I attach to my CustomOverlay:

http://texttopng.azurewebsites.net/Home/TextToPng?text=48

And here is related stack overflow question:

https://stackoverflow.com/questions/59718486/marker-overlays-must-be-png-jpg-or-webp-while-using-mapboxstatic-api

It worked before, now it stopped working. Why?