Reason for the errors (Service Unavailable - Zero size object) and (Session contains no certificate)

Can someone give examples of explanations/causes for the following errors when calling the activities endpoint in Oracle Field Service. The occurrence is rare but would still like to know the root cause or possible causes.

End point API:

https://xxxx.fs.ocs.oraclecloud.com/rest/ofscCore/v1/activities

Errors

1) Create Activity - Service Unavailable - Zero size object (The server is temporarily unable to service your request. Please try again later.)

2) Create Activity - org.apache.commons.httpclient.HttpException: Session contains no certificates - Untrusted