When investigating what can cause a fetch API with limited information you can click the "Show HTTP events" button on one of the sessions and look for http calls that might cause a fetch call to be made.
In the above case, the /api/subscription/new endpoint is called, it could be the case that a webhook is attached to that event that uses the fetch api internally.
In some cases however, it is possible that we extract valuable information from the failure and are able to point you towards the location of the failure in your code. As seen in the picture below, you can extract the SourceURL of the failure by clicking on the details tab.
You can then copy that URL and paste the link in another tab. Then you can follow the guide here to debug the error further.