For anyone still experiencing this, the issue is that the request contains the “nexposeCCSessionID” cookie.
When making an API call, the only authentication needed is the “Authorization” header, if at some point the IVM server returns that session ID cookie, then any requests you send with it will error with HTTP 406 “Not Acceptable”.
Delete the nexposeCCSessionID cookie from your request to fix this. Or as the other commenters mention, completely reinstall/update whichever software you were using, which should also have the effect of clearing the cookie