Hi Melanie & Jack
Melanie thanks for sharing the thumbnail image and thanks both for the additional info.
I've done some testing using your image (Stadt_für_Alle+-+Kopie.png). After publishing, the thumbnail disappears from the survey's tile in the Connect gallery. The image appears as expected in the field app, Survey123 website and ArcGIS Online items.
If I remove the spaces, special characters and symbols (+ plus sign and ü umlaut) from the filename, replace the thumbnail and publish again the thumbnail is displayed in the Connect gallery as expected. I've logged this issue in our system and will post any updates to this thread.
In the meantime, the workaround is to rename the image file and publish the survey again; or download the survey to Connect (which will refresh the local thumbnail file with the online version). Could you please give this a try and let me know how it goes?
Jack Palmer - just to assist with our investigation, does your image filename contain any spaces, special characters or symbols?
Best regards,
Jim