When using Curl, add "-v" to see the header. For example: Authentication with API token without (necessary) use of https:// leads to:
curl -v "http://combitreportserver/api/v1/....?clientid=abc&clienttoken=123
< HTTP/1.1 403 ERR_SSL_REQUIRED
< Cache-Control: private
< Date: Tue, 14 Apr 2015 11:04:04 GMT
< Content-Length: 0
The error code (i.e. "ERR_SSL_REQUIRED") will then be displayed here.