The Test Repository hierarchy endpoint, returns always the error 503 Service Unavailable

The endpoint that returns the Test Repository folders hierarchy, returns always 503 Service Unavailable.

Endpoint: GET /rest/raven/1.0/api/testrepository/<projectKey>/folders

Pre-condition: Have installed the Xray 3.3.7 or 3.3.8.

Steps to Reproduce

Step	Result
Use the endpoint GET /rest/raven/1.0/api/testrepository/<projectKey>/folders