Hey @Benjamin123, this is an unsupported and undocumented API used by the UI which can change at any point in time. I try to make this clear whenever it comes up on the forum, apologies if I have missed it in your case.
That said, here is a large warning on the changelog about this:
Upgrading to any n8n version that includes user management will cause calls to the frontend API to break.
I suspect you could work around this by performing the requests made by the frontend when logging in and then use the auth cookie for your workflows request, but I haven’t tried this yet and won’t be able to in the next days from the looks of it.
So if you don’t want to do the discovery work yourself your best bet would be to use a version of n8n without user management for now and wait for a public REST API to be built and released. This will make things much simpler 