One of the best practices of using the Clarizen API is calling GetServerDefinition first to get the correct base URL to use with the API.
The end point address you specified is not an address we use for the API, and though it may work it may have issues.
I suggest you start with https://apie1.clarizen.com/v2.0/services/ and call GetServerDefinition first thing in your application to verify/change the base url if required.
Check the REST API documentation for further information.
An HTTP 503 error could also suggest service disruption, so we'd better make sure it isn't an issue of using the wrong address.
Hope this helps,