Create Event Subscription 404 and subsequent 409

Hi all!

I started trying out the NetSapiens Sandbox and API lately and ran into my first issue.

I want to create a call event subscription for the user I got when creating the Sandbox.
However, this gives me a 404 - Resource not found error. Trying the exact same request again gives me a 409 - Subscription ID already Exists or matching set of parameters already exists. Trying out the 'Read Event Subscriptions' endpoint shows me no subscriptions have been made, what's going on here?

Furthermore, doing the same request but leaving out the 'user' parameter is succesful, so creating a subscription for the whole domain is fine.

I've read to all of the relevant guides and api documentation, but I cannot find any details on what kind of configuration I could be missing. I have no experience with the Netsapiens platform yet, so I might be missing something simple. Do I need to create a SIP trunk first? A route?

Thanks in advance for any reply.