Flutter Web: Loading data from URL fails

When trying to fetch image data inside a custom ImageProvider from an URL, I get an error: ClientException: XMLHttpRequest error., uri=
https:///api/v1/images//thumbnail-squared The host however has CORS enabled and returns a header "access-control-allow-origin" -> "*". The URL redirects a 302 to a signed google cloud storage file however. Might this be the issue?