I have extended the REST API as described in the official documentation, and I can access my newly created API route from my frontend siteaccess, so e.g.

http://frontend.ezpublish.local/api/ezp/v2/my_rest_bundle/hello

When I call the same route from the legacy backend

http://backend.ezpublish.local/api/ezp/v2/my_rest_bundle/hello

I get a kernel(20) error page.

The default REST API routes provided by eZ Publish out of the box work, so e.g.

http://frontend.ezpublish.local/api/ezp/v2/content/types
and
http://backend.ezpublish.local/api/ezp/v2/content/types

both return the expected result.

I would like to be able to access my custom REST API from the legacy backend, too.