Unable to set Service Request Template with API?

I have an incoming e-mail that I want to handle, right now we have a custom trigger that waits for the e-mail and runs a powershell script to change values, it's been working fine for several years.

Now I'm (for the first time) trying to apply a Service Request Template to the incoming e-mail (instead of a Incident Template). The Service Request has a Life Cycle connected (if that makes any difference).

The remote server returned an error: (400) Bad Request. - {"response_status":{"status_code":4000,"messages":[{"status_code":4001,"field":"template","type":"failed","message":"Invalid Input"}],"status":"failed"}}

If I uncomment the template, the script works again.

I've tried setting the correct Status (to match the life cycle) but that doesn't help either.