Ok i think I know what is the root cause and got a workaround for it.
So… I recreated the issue with the same theme, plugins etc on my local environment and I managed to start the popup from a template.
Moreover, on my website I noticed 403 coming up from doing a POST request:
https://silverjoy.pl/wp-json/wp/v2/media/
Again I checked on my local env if brave is trying to set up anything to my media and apparently after you choose the template with a picture it will try to upload it to media with exactly the same POST request.
Since the images you have in the templates you are putting under the same name into the media so i copied the picture: 52e2d04b4857af14f6da8c7dda793179113ac3e45656734d7c277fd09f_640.jpg
into my media and it worked :P.
Still seems weird that once it is able to do the request and once not. Since its 403 maybe because you are trying to put something into the media you are being blocked and the request blocked the UI which ends up in forever spinning loop ??
Hope that helps.
Regards,
?ukasz W?odkowski