Marketing activity flow problem

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I am trying to create a marketing activity extension for our app, but cannot get it working. I have already made the extension and the server endpoint POST:preload_form_data. When the shop owner clicks "Create campaign", that endpoint is called by Shopify, as expected. My app responds with 200:OK and payload

	form_data: {
as there is no options that should be populated on the fly. At this point I come to a dead end, as the client throws this error:
Screenshot 2020-09-09 at 13.37.10.png
while our app does not receive any other request from Shopify to any other endpoints, so I cannot understand what this error refers to as a non-existing page.
Any suggestions what I may missed is highly appreciated.
Kind regards,